Michelson: the language of Smart Contracts in Dune¶

The language is stack-based, with high level data types and primitives and strict static type checking. Its design cherry picks traits from several language families. Vigilant readers will notice direct references to Forth, Scheme, ML and Cat.

A Michelson program is a series of instructions that are run in sequence: each instruction receives as input the stack resulting of the previous instruction, and rewrites it for the next one. The stack contains both immediate values and heap allocated structures. All values are immutable and garbage collected.

A Michelson program receives as input a stack containing a single pair whose first element is an input value and second element the content of a storage space. It must return a stack containing a single pair whose first element is a list of internal operations, and second element the new contents of the storage space. Alternatively, a Michelson program can fail, explicitly using a specific opcode, or because something went wrong that could not be caught by the type system (e.g. division by zero, gas exhaustion).

The types of the input, output and storage are fixed and monomorphic, and the program is typechecked before being introduced into the system. No smart contract execution can fail because an instruction has been executed on a stack of unexpected length or contents.

This specification gives the complete instruction set, type system and semantics of the language. It is meant as a precise reference manual, not an easy introduction. Even though, some examples are provided at the end of the document and can be read first or at the same time as the specification.

Semantics¶

This specification gives a detailed formal semantics of the Michelson language. It explains in a symbolic way the computation performed by the Michelson interpreter on a given program and initial stack to produce the corresponding resulting stack. The Michelson interpreter is a pure function: it only builds a result stack from the elements of an initial one, without affecting its environment. This semantics is then naturally given in what is called a big step form: a symbolic definition of a recursive reference interpreter. This definition takes the form of a list of rules that cover all the possible inputs of the interpreter (program and stack), and describe the computation of the corresponding resulting stacks.

Rules form and selection¶

The rules have the main following form.

> (syntax pattern) / (initial stack pattern)  =>  (result stack pattern)
    iff (conditions)
    where (recursions)
    and (more recursions)

The left hand side of the => sign is used for selecting the rule. Given a program and an initial stack, one (and only one) rule can be selected using the following process. First, the toplevel structure of the program must match the syntax pattern. This is quite simple since there is only a few non trivial patterns to deal with instruction sequences, and the rest is made of trivial pattern that match one specific instruction. Then, the initial stack must match the initial stack pattern. Finally, some rules add extra conditions over the values in the stack that follow the iff keyword. Sometimes, several rules may apply in a given context. In this case, the one that appears first in this specification is to be selected. If no rule applies, the result is equivalent to the one for the explicit FAILWITH instruction. This case does not happen on well-typed programs, as explained in the next section.

The right hand side describes the result of the interpreter if the rule applies. It consists in a stack pattern, whose parts are either constants, or elements of the context (program and initial stack) that have been named on the left hand side of the => sign.

Recursive rules (big step form)¶

Sometimes, the result of interpreting a program is derived from the result of interpreting another one (as in conditionals or function calls). In these cases, the rule contains a clause of the following form.

where (intermediate program) / (intermediate stack)  =>  (partial result)

This means that this rules applies in case interpreting the intermediate state on the left gives the pattern on the right.

The left hand sign of the => sign is constructed from elements of the initial state or other partial results, and the right hand side identify parts that can be used to build the result stack of the rule.

If the partial result pattern does not actually match the result of the interpretation, then the result of the whole rule is equivalent to the one for the explicit FAILWITH instruction. Again, this case does not happen on well-typed programs, as explained in the next section.

Format of patterns¶

Code patterns are of one of the following syntactical forms.

INSTR (an uppercase identifier) is a simple instruction (e.g. DROP).
INSTR (arg) ... is a compound instruction, whose arguments can be code, data or type patterns (e.g. PUSH nat 3).
{ (instr) ; ... } is a possibly empty sequence of instructions, (e.g. IF { SWAP ; DROP } { DROP }), nested sequences can drop the braces.
name is a pattern that matches any program and names a part of the matched program that can be used to build the result.
_ is a pattern that matches any instruction.

Stack patterns are of one of the following syntactical forms.

[FAILED] is the special failed state.
[] is the empty stack.
(top) : (rest) is a stack whose top element is matched by the data pattern (top) on the left, and whose remaining elements are matched by the stack pattern (rest) on the right (e.g. x : y : rest).
name is a pattern that matches any stack and names it in order to use it to build the result.
_ is a pattern that matches any stack.

Data patterns are of one of the following syntactical forms.

integer/natural number literals, (e.g. 3).
string literals, (e.g. "contents").
raw byte sequence literals (e.g. 0xABCDEF42).
Tag (capitalized) is a symbolic constant, (e.g. Unit, True, False).
(Tag (arg) ...) tagged constructed data, (e.g. (Pair 3 4)).
a code pattern for first class code values.
name to name a value in order to use it to build the result.
_ to match any value.

The domain of instruction names, symbolic constants and data constructors is fixed by this specification. Michelson does not let the programmer introduce its own types.

Be aware that the syntax used in the specification may differ a bit from the concrete syntax. In particular some instructions are annotated with types that are not present in the concrete language because they are synthesized by the typechecker.

Shortcuts¶

Sometimes, it is easier to think (and shorter to write) in terms of program rewriting than in terms of big step semantics. When it is the case, and when both are equivalents, we write rules of the form:

p / S  =>  S''
where   p' / S'  =>  S''

using the following shortcut:

p / S  =>  p' / S'

The concrete language also has some syntax sugar to group some common sequences of operations as one. This is described in this specification using a simple regular expression style recursive instruction rewriting.

Introduction to the type system and notations¶

This specification describes a type system for Michelson. To make things clear, in particular to readers that are not accustomed to reading formal programming language specifications, it does not give a typechecking or inference algorithm. It only gives an intentional definition of what we consider to be well-typed programs. For each syntactical form, it describes the stacks that are considered well-typed inputs, and the resulting outputs.

The type system is sound, meaning that if a program can be given a type, then if run on a well-typed input stack, the interpreter will never apply an interpretation rule on a stack of unexpected length or contents. Also, it will never reach a state where it cannot select an appropriate rule to continue the execution. Well-typed programs do not block, and do not go wrong.

Type notations¶

The specification introduces notations for the types of values, terms and stacks. Apart from a subset of value types that appear in the form of type annotations in some places throughout the language, it is important to understand that this type language only exists in the specification.

A stack type can be written:

[] for the empty stack.
(top) : (rest) for the stack whose first value has type (top) and queue has stack type (rest).

Instructions, programs and primitives of the language are also typed, their types are written:

(type of stack before) -> (type of stack after)

The types of values in the stack are written:

identifier for a primitive data-type (e.g. bool).
identifier (arg) for a parametric data-type with one parameter type (arg) (e.g. list nat).
identifier (arg) ... for a parametric data-type with several parameters (e.g. map string int).
[ (type of stack before) -> (type of stack after) ] for a code quotation, (e.g. [ int : int : [] -> int : [] ]).
lambda (arg) (ret) is a shortcut for [ (arg) : [] -> (ret) : [] ].

Meta type variables¶

The typing rules introduce meta type variables. To be clear, this has nothing to do with polymorphism, which Michelson does not have. These variables only live at the specification level, and are used to express the consistency between the parts of the program. For instance, the typing rule for the IF construct introduces meta variables to express that both branches must have the same type.

Here are the notations for meta type variables:

'a for a type variable.
'A for a stack type variable.
_ for an anonymous type or stack type variable.

Typing rules¶

The system is syntax directed, which means here that it defines a single typing rule for each syntax construct. A typing rule restricts the type of input stacks that are authorized for this syntax construct, links the output type to the input type, and links both of them to the subexpressions when needed, using meta type variables.

Typing rules are of the form:

(syntax pattern)
:: (type of stack before) -> (type of stack after) [rule-name]
   iff (premises)

Where premises are typing requirements over subprograms or values in the stack, both of the form (x) :: (type), meaning that value (x) must have type (type).

A program is shown well-typed if one can find an instance of a rule that applies to the toplevel program expression, with all meta type variables replaced by non variable type expressions, and of which all type requirements in the premises can be proven well-typed in the same manner. For the reader unfamiliar with formal type systems, this is called building a typing derivation.

Here is an example typing derivation on a small program that computes (x+5)*10 for a given input x, obtained by instantiating the typing rules for instructions PUSH, ADD and for the sequence, as found in the next sections. When instantiating, we replace the iff with by.

{ PUSH nat 5 ; ADD ; PUSH nat 10 ; SWAP ; MUL }
:: [ nat : [] -> nat : [] ]
   by { PUSH nat 5 ; ADD }
      :: [ nat : [] -> nat : [] ]
         by PUSH nat 5
            :: [ nat : [] -> nat : nat : [] ]
               by 5 :: nat
        and ADD
            :: [ nat : nat : [] -> nat : [] ]
  and { PUSH nat 10 ; SWAP ; MUL }
      :: [ nat : [] -> nat : [] ]
         by PUSH nat 10
            :: [ nat : [] -> nat : nat : [] ]
               by 10 :: nat
        and { SWAP ; MUL }
            :: [ nat : nat : [] -> nat : [] ]
               by SWAP
                  :: [ nat : nat : [] -> nat : nat : [] ]
              and MUL
                  :: [ nat : nat : [] -> nat : [] ]

Producing such a typing derivation can be done in a number of manners, such as unification or abstract interpretation. In the implementation of Michelson, this is done by performing a recursive symbolic evaluation of the program on an abstract stack representing the input type provided by the programmer, and checking that the resulting symbolic stack is consistent with the expected result, also provided by the programmer.

Side note¶

As with most type systems, it is incomplete. There are programs that cannot be given a type in this type system, yet that would not go wrong if executed. This is a necessary compromise to make the type system usable. Also, it is important to remember that the implementation of Michelson does not accept as many programs as the type system describes as well-typed. This is because the implementation uses a simple single pass typechecking algorithm, and does not handle any form of polymorphism.

Core data types and notations¶

string, nat, int and bytes: The core primitive constant types.
bool: The type for booleans whose values are True and False.
unit: The type whose only value is Unit, to use as a placeholder when some result or parameter is non necessary. For instance, when the only goal of a contract is to update its storage.
list (t): A single, immutable, homogeneous linked list, whose elements are of type (t), and that we note {} for the empty list or { first ; ... }. In the semantics, we use chevrons to denote a subsequence of elements. For instance { head ; <tail> }.
pair (l) (r): A pair of values a and b of types (l) and (r), that we write (Pair a b).
option (t): Optional value of type (t) that we note None or (Some v).
or (l) (r): A union of two types: a value holding either a value a of type (l) or a value b of type (r), that we write (Left a) or (Right b).
set (t): Immutable sets of values of type (t) that we note as lists { item ; ... }, of course with their elements unique, and sorted.
map (k) (t): Immutable maps from keys of type (k) of values of type (t) that we note { Elt key value ; ... }, with keys sorted.
big_map (k) (t): Lazily deserialized maps from keys of type (k) of values of type (t) that we note { Elt key value ; ... }, with keys sorted. These maps should be used if you intend to store large amounts of data in a map. They have higher gas costs than standard maps as data is lazily deserialized. You are limited to a single big_map per program, which must appear on the left hand side of a pair in the contract’s storage.

Core instructions¶

Control structures¶

FAILWITH: Explicitly abort the current program.

‘a :: _ -> _

This special instruction aborts the current program exposing the top of the stack in its error message (first rule below). It makes the output useless since all subsequent instructions will simply ignore their usual semantics to propagate the failure up to the main result (second rule below). Its type is thus completely generic.

> FAILWITH / a : _  =>  [FAILED]
> _ / [FAILED]  =>  [FAILED]

{}: Empty sequence.

:: 'A   ->   'A

> {} / SA  =>  SA

{ I ; C }: Sequence.

:: 'A   ->   'C
   iff   I :: [ 'A -> 'B ]
         C :: [ 'B -> 'C ]

> I ; C / SA  =>  SC
    where   I / SA  =>  SB
    and   C / SB  =>  SC

IF bt bf: Conditional branching.

:: bool : 'A   ->   'B
   iff   bt :: [ 'A -> 'B ]
         bf :: [ 'A -> 'B ]

> IF bt bf / True : S  =>  bt / S
> IF bt bf / False : S  =>  bf / S

LOOP body: A generic loop.

:: bool : 'A   ->   'A
   iff   body :: [ 'A -> bool : 'A ]

> LOOP body / True : S  =>  body ; LOOP body / S
> LOOP body / False : S  =>  S

LOOP_LEFT body: A loop with an accumulator.

:: (or 'a 'b) : 'A   ->  'b : 'A
   iff   body :: [ 'a : 'A -> (or 'a 'b) : 'A ]

> LOOP_LEFT body / (Left a) : S  =>  body ; LOOP_LEFT body / a : S
> LOOP_LEFT body / (Right b) : S  =>  b : S

DIP code: Runs code protecting the top of the stack.

:: 'b : 'A   ->   'b : 'C
   iff   code :: [ 'A -> 'C ]

> DIP code / x : S  =>  x : S'
    where    code / S  =>  S'

EXEC: Execute a function from the stack.

:: 'a : lambda 'a 'b : 'C   ->   'b : 'C

> EXEC / a : f : S  =>  r : S
    where f / a : []  =>  r : []

Stack operations¶

DROP: Drop the top element of the stack.

:: _ : 'A   ->   'A

> DROP / _ : S  =>  S

DUP: Duplicate the top of the stack.

:: 'a : 'A   ->   'a : 'a : 'A

> DUP / x : S  =>  x : x : S

SWAP: Exchange the top two elements of the stack.

:: 'a : 'b : 'A   ->   'b : 'a : 'A

> SWAP / x : y : S  =>  y : x : S

PUSH 'a x: Push a constant value of a given type onto the stack.

:: 'A   ->   'a : 'A
   iff   x :: 'a

> PUSH 'a x / S  =>  x : S

UNIT: Push a unit value onto the stack.

:: 'A   ->   unit : 'A

> UNIT / S  =>  Unit : S

LAMBDA 'a 'b code: Push a lambda with given parameter and return types onto the stack.

:: 'A ->  (lambda 'a 'b) : 'A

> LAMBDA _ _ code / S  =>  code : S

Generic comparison¶

Comparison only works on a class of types that we call comparable. A COMPARE operation is defined in an ad hoc way for each comparable type, but the result of compare is always an int, which can in turn be checked in a generic manner using the following combinators. The result of COMPARE is 0 if the top two elements of the stack are equal, negative if the first element in the stack is less than the second, and positive otherwise.

EQ: Checks that the top of the stack EQuals zero.

:: int : 'S   ->   bool : 'S

> EQ / 0 : S  =>  True : S
> EQ / v : S  =>  False : S
    iff v <> 0

NEQ: Checks that the top of the stack does Not EQual zero.

:: int : 'S   ->   bool : 'S

> NEQ / 0 : S  =>  False : S
> NEQ / v : S  =>  True : S
    iff v <> 0

LT: Checks that the top of the stack is Less Than zero.

:: int : 'S   ->   bool : 'S

> LT / v : S  =>  True : S
    iff  v < 0
> LT / v : S  =>  False : S
    iff v >= 0

GT: Checks that the top of the stack is Greater Than zero.

:: int : 'S   ->   bool : 'S

> GT / v : S  =>  C / True : S
    iff  v > 0
> GT / v : S  =>  C / False : S
    iff v <= 0

LE: Checks that the top of the stack is Less Than of Equal to zero.

:: int : 'S   ->   bool : 'S

> LE / v : S  =>  True : S
    iff  v <= 0
> LE / v : S  =>  False : S
    iff v > 0

GE: Checks that the top of the stack is Greater Than of Equal to zero.

:: int : 'S   ->   bool : 'S

> GE / v : S  =>  True : S
    iff  v >= 0
> GE / v : S  =>  False : S
    iff v < 0

Operations¶

Operations on booleans¶

OR

:: bool : bool : 'S   ->   bool : 'S

> OR / x : y : S  =>  (x | y) : S

AND

:: bool : bool : 'S   ->   bool : 'S

> AND / x : y : S  =>  (x & y) : S

XOR

:: bool : bool : 'S   ->   bool : 'S

> XOR / x : y : S  =>  (x ^ y) : S

NOT

:: bool : 'S   ->   bool : 'S

> NOT / x : S  =>  ~x : S

Operations on integers and natural numbers¶

Integers and naturals are arbitrary-precision, meaning the only size limit is fuel.

NEG

:: int : 'S   ->   int : 'S
:: nat : 'S   ->   int : 'S

> NEG / x : S  =>  -x : S

ABS

:: int : 'S   ->   nat : 'S

> ABS / x : S  =>  abs (x) : S

ADD

:: int : int : 'S   ->   int : 'S
:: int : nat : 'S   ->   int : 'S
:: nat : int : 'S   ->   int : 'S
:: nat : nat : 'S   ->   nat : 'S

> ADD / x : y : S  =>  (x + y) : S

SUB

:: int : int : 'S   ->   int : 'S
:: int : nat : 'S   ->   int : 'S
:: nat : int : 'S   ->   int : 'S
:: nat : nat : 'S   ->   int : 'S

> SUB / x : y : S  =>  (x - y) : S

MUL

:: int : int : 'S   ->   int : 'S
:: int : nat : 'S   ->   int : 'S
:: nat : int : 'S   ->   int : 'S
:: nat : nat : 'S   ->   nat : 'S

> MUL / x : y : S  =>  (x * y) : S

EDIV Perform Euclidian division

:: int : int : 'S   ->   option (pair int nat) : 'S
:: int : nat : 'S   ->   option (pair int nat) : 'S
:: nat : int : 'S   ->   option (pair int nat) : 'S
:: nat : nat : 'S   ->   option (pair nat nat) : 'S

> EDIV / x : 0 : S  =>  None : S
> EDIV / x : y : S  =>  Some (Pair (x / y) (x % y)) : S
    iff y <> 0

Bitwise logical operators are also available on unsigned integers.

OR

:: nat : nat : 'S   ->   nat : 'S

> OR / x : y : S  =>  (x | y) : S

AND (also available when the top operand is signed)

:: nat : nat : 'S   ->   nat : 'S
:: int : nat : 'S   ->   nat : 'S

> AND / x : y : S  =>  (x & y) : S

XOR

:: nat : nat : 'S   ->   nat : 'S

> XOR / x : y : S  =>  (x ^ y) : S

NOT The return type of NOT is an int and not a nat. This is because the sign is also negated. The resulting integer is computed using two’s complement. For instance, the boolean negation of 0 is -1. To get a natural back, a possibility is to use AND with an unsigned mask afterwards.

:: nat : 'S   ->   int : 'S
:: int : 'S   ->   int : 'S

> NOT / x : S  =>  ~x : S

LSL

:: nat : nat : 'S   ->   nat : 'S

> LSL / x : s : S  =>  (x << s) : S
    iff   s <= 256
> LSL / x : s : S  =>  [FAILED]
    iff   s > 256

LSR

:: nat : nat : 'S   ->   nat : 'S

> LSR / x : s : S  =>  (x >> s) : S
    iff   s <= 256
> LSR / x : s : S  =>  [FAILED]
    iff   s > 256

COMPARE: Integer/natural comparison

:: int : int : 'S   ->   int : 'S
:: nat : nat : 'S   ->   int : 'S

> COMPARE / x : y : S  =>  -1 : S
    iff x < y
> COMPARE / x : y : S  =>  0 : S
    iff x = y
> COMPARE / x : y : S  =>  1 : S
    iff x > y

Operations on strings¶

Strings are mostly used for naming things without having to rely on external ID databases. They are restricted to the printable subset of 7-bit ASCII, plus some escaped characters (see section on constants). So what can be done is basically use string constants as is, concatenate or splice them, and use them as keys.

CONCAT: String concatenation.

:: string : string : 'S   -> string : 'S

> CONCAT / s : t : S  =>  (s ^ t) : S

:: string list : 'S   -> string : 'S

> CONCAT / {} : S  =>  "" : S
> CONCAT / { s ; <ss> } : S  =>  (s ^ r) : S
   where CONCAT / { <ss> } : S  =>  r : S

SIZE: number of characters in a string.

:: string : 'S   ->   nat : 'S

SLICE: String access.

:: nat : nat : string : 'S   ->  option string : 'S

> SLICE / offset : length : s : S  =>  Some ss : S
   where ss is the substring of s at the given offset and of the given length
     iff offset and (offset + length) are in bounds
> SLICE / offset : length : s : S  =>  None  : S
     iff offset or (offset + length) are out of bounds

COMPARE: Lexicographic comparison.

:: string : string : 'S   ->   int : 'S

> COMPARE / s : t : S  =>  -1 : S
    iff s < t
> COMPARE / s : t : S  =>  0 : S
    iff s = t
> COMPARE / s : t : S  =>  1 : S
    iff s > t

Operations on pairs¶

PAIR: Build a pair from the stack’s top two elements.

:: 'a : 'b : 'S   ->   pair 'a 'b : 'S

> PAIR / a : b : S  =>  (Pair a b) : S

CAR: Access the left part of a pair.

:: pair 'a _ : 'S   ->   'a : 'S

> CAR / (Pair a _) : S  =>  a : S

CDR: Access the right part of a pair.

:: pair _ 'b : 'S   ->   'b : 'S

> CDR / (Pair _ b) : S  =>  b : S

Operations on sets¶

EMPTY_SET 'elt: Build a new, empty set for elements of a given type.

The 'elt type must be comparable (the COMPARE primitive must be defined over it).

:: 'S   ->   set 'elt : 'S

> EMPTY_SET _ / S  =>  {} : S

MEM: Check for the presence of an element in a set.

:: 'elt : set 'elt : 'S   ->  bool : 'S

> MEM / x : {} : S  =>  false : S
> MEM / x : { hd ; <tl> } : S  =>  r : S
    iff COMPARE / x : hd : []  =>  1 : []
    where MEM / x : { <tl> } : S  =>  r : S
> MEM / x : { hd ; <tl> } : S  =>  true : S
    iff COMPARE / x : hd : []  =>  0 : []
> MEM / x : { hd ; <tl> } : S  =>  false : S
    iff COMPARE / x : hd : []  =>  -1 : []

UPDATE: Inserts or removes an element in a set, replacing a previous value.

:: 'elt : bool : set 'elt : 'S   ->   set 'elt : 'S

> UPDATE / x : false : {} : S  =>  {} : S
> UPDATE / x : true : {} : S  =>  { x } : S
> UPDATE / x : v : { hd ; <tl> } : S  =>  { hd ; <tl'> } : S
    iff COMPARE / x : hd : []  =>  1 : []
    where UPDATE / x : v : { <tl> } : S  =>  { <tl'> } : S
> UPDATE / x : false : { hd ; <tl> } : S  =>  { <tl> } : S
    iff COMPARE / x : hd : []  =>  0 : []
> UPDATE / x : true : { hd ; <tl> } : S  =>  { hd ; <tl> } : S
    iff COMPARE / x : hd : []  =>  0 : []
> UPDATE / x : false : { hd ; <tl> } : S  =>  { hd ; <tl> } : S
    iff COMPARE / x : hd : []  =>  -1 : []
> UPDATE / x : true : { hd ; <tl> } : S  =>  { x ; hd ; <tl> } : S
    iff COMPARE / x : hd : []  =>  -1 : []

ITER body: Apply the body expression to each element of a set. The body sequence has access to the stack.

:: (set 'elt) : 'A   ->  'A
   iff body :: [ 'elt : 'A -> 'A ]

> ITER body / {} : S  =>  S
> ITER body / { hd ; <tl> } : S  =>  body; ITER body / hd : { <tl> } : S

SIZE: Get the cardinality of the set.

:: set 'elt : 'S -> nat : 'S

> SIZE / {} : S  =>  0 : S
> SIZE / { _ ; <tl> } : S  =>  1 + s : S
    where SIZE / { <tl> } : S  =>  s : S

Operations on maps¶

EMPTY_MAP 'key 'val: Build a new, empty map from keys of a given type to values of another given type.

The 'key type must be comparable (the COMPARE primitive must be defined over it).

:: 'S -> map 'key 'val : 'S

> EMPTY_MAP _ _ / S  =>  {} : S

GET: Access an element in a map, returns an optional value to be checked with IF_SOME.

:: 'key : map 'key 'val : 'S   ->   option 'val : 'S

> GET / x : {} : S  =>  None : S
> GET / x : { Elt k v ; <tl> } : S  =>  opt_y : S
    iff COMPARE / x : k : []  =>  1 : []
    where GET / x : { <tl> } : S  =>  opt_y : S
> GET / x : { Elt k v ; <tl> } : S  =>  Some v : S
    iff COMPARE / x : k : []  =>  0 : []
> GET / x : { Elt k v ; <tl> } : S  =>  None : S
    iff COMPARE / x : k : []  =>  -1 : []

MEM: Check for the presence of a binding for a key in a map.

:: 'key : map 'key 'val : 'S   ->  bool : 'S

> MEM / x : {} : S  =>  false : S
> MEM / x : { Elt k v ; <tl> } : S  =>  r : S
    iff COMPARE / x : k : []  =>  1 : []
    where MEM / x : { <tl> } : S  =>  r : S
> MEM / x : { Elt k v ; <tl> } : S  =>  true : S
    iff COMPARE / x : k : []  =>  0 : []
> MEM / x : { Elt k v ; <tl> } : S  =>  false : S
    iff COMPARE / x : k : []  =>  -1 : []

UPDATE: Assign or remove an element in a map.

:: 'key : option 'val : map 'key 'val : 'S   ->   map 'key 'val : 'S

> UPDATE / x : None : {} : S  =>  {} : S
> UPDATE / x : Some y : {} : S  =>  { Elt x y } : S
> UPDATE / x : opt_y : { Elt k v ; <tl> } : S  =>  { Elt k v ; <tl'> } : S
    iff COMPARE / x : k : []  =>  1 : []
          where UPDATE / x : opt_y : { <tl> } : S  =>  { <tl'> } : S
> UPDATE / x : None : { Elt k v ; <tl> } : S  =>  { <tl> } : S
    iff COMPARE / x : k : []  =>  0 : []
> UPDATE / x : Some y : { Elt k v ; <tl> } : S  =>  { Elt k y ; <tl> } : S
    iff COMPARE / x : k : []  =>  0 : []
> UPDATE / x : None : { Elt k v ; <tl> } : S  =>  { Elt k v ; <tl> } : S
    iff COMPARE / x : k : []  =>  -1 : []
> UPDATE / x : Some y : { Elt k v ; <tl> } : S  =>  { Elt x y ; Elt k v ; <tl> } : S
    iff COMPARE / x : k : []  =>  -1 : []

MAP body: Apply the body expression to each element of a map. The body sequence has access to the stack.

:: (map 'key 'val) : 'A   ->  (map 'key 'b) : 'A
   iff   body :: [ (pair 'key 'val) : 'A -> 'b : 'A ]

> MAP body / {} : S  =>  {} : S
> MAP body / { Elt k v ; <tl> } : S  =>  { Elt k (body (Pair k v)) ; <tl'> } : S
    where MAP body / { <tl> } : S  =>  { <tl'> } : S

ITER body: Apply the body expression to each element of a map. The body sequence has access to the stack.

:: (map 'elt 'val) : 'A   ->  'A
   iff   body :: [ (pair 'elt 'val : 'A) -> 'A ]

> ITER body / {} : S  =>  S
> ITER body / { Elt k v ; <tl> } : S  =>  body ; ITER body / (Pair k v) : { <tl> } : S

SIZE: Get the cardinality of the map.

:: map 'key 'val : 'S -> nat : 'S

> SIZE / {} : S  =>  0 : S
> SIZE / { _ ; <tl> } : S  =>  1 + s : S
    where  SIZE / { <tl> } : S  =>  s : S

Operations on `big_maps`¶

The behavior of these operations is the same as if they were normal maps, except that under the hood, the elements are loaded and deserialized on demand.

GET: Access an element in a big_map, returns an optional value to be checked with IF_SOME.

:: 'key : big_map 'key 'val : 'S   ->   option 'val : 'S

MEM: Check for the presence of an element in a big_map.

:: 'key : big_map 'key 'val : 'S   ->  bool : 'S

UPDATE: Assign or remove an element in a big_map.

:: 'key : option 'val : big_map 'key 'val : 'S   ->   big_map 'key 'val : 'S

Operations on optional values¶

SOME: Pack a present optional value.

:: 'a : 'S   ->   option 'a : 'S

> SOME / v : S  =>  (Some v) : S

NONE 'a: The absent optional value.

:: 'S   ->   option 'a : 'S

> NONE / v : S  =>  None : S

IF_NONE bt bf: Inspect an optional value.

:: option 'a : 'S   ->   'b : 'S
   iff   bt :: [ 'S -> 'b : 'S]
         bf :: [ 'a : 'S -> 'b : 'S]

> IF_NONE bt bf / (None) : S  =>  bt / S
> IF_NONE bt bf / (Some a) : S  =>  bf / a : S

Operations on unions¶

LEFT 'b: Pack a value in a union (left case).

:: 'a : 'S   ->   or 'a 'b : 'S

> LEFT / v : S  =>  (Left v) : S

RIGHT 'a: Pack a value in a union (right case).

:: 'b : 'S   ->   or 'a 'b : 'S

> RIGHT / v : S  =>  (Right v) : S

IF_LEFT bt bf: Inspect a value of a union.

:: or 'a 'b : 'S   ->   'c : 'S
   iff   bt :: [ 'a : 'S -> 'c : 'S]
         bf :: [ 'b : 'S -> 'c : 'S]

> IF_LEFT bt bf / (Left a) : S  =>  bt / a : S
> IF_LEFT bt bf / (Right b) : S  =>  bf / b : S

IF_RIGHT bt bf: Inspect a value of a union.

:: or 'a 'b : 'S   ->   'c : 'S
   iff   bt :: [ 'b : 'S -> 'c : 'S]
         bf :: [ 'a : 'S -> 'c : 'S]

> IF_RIGHT bt bf / (Right b) : S  =>  bt / b : S
> IF_RIGHT bt bf / (Left a) : S  =>  bf / a : S

Operations on lists¶

CONS: Prepend an element to a list.

:: 'a : list 'a : 'S   ->   list 'a : 'S

> CONS / a : { <l> } : S  =>  { a ; <l> } : S

NIL 'a: The empty list.

:: 'S   ->   list 'a : 'S

> NIL / S  =>  {} : S

IF_CONS bt bf: Inspect a list.

:: list 'a : 'S   ->   'b : 'S
   iff   bt :: [ 'a : list 'a : 'S -> 'b : 'S]
         bf :: [ 'S -> 'b : 'S]

> IF_CONS bt bf / { a ; <rest> } : S  =>  bt / a : { <rest> } : S
> IF_CONS bt bf / {} : S  =>  bf / S

MAP body: Apply the body expression to each element of the list. The body sequence has access to the stack.

:: (list 'elt) : 'A   ->  (list 'b) : 'A
   iff   body :: [ 'elt : 'A -> 'b : 'A ]

> MAP body / { a ; <rest> } : S  =>  { body a ; <rest'> } : S
    where MAP body / { <rest> } : S  =>  { <rest'> } : S
> MAP body / {} : S  =>  {} : S

SIZE: Get the number of elements in the list.

:: list 'elt : 'S -> nat : 'S

> SIZE / { _ ; <rest> } : S  =>  1 + s : S
    where  SIZE / { <rest> } : S  =>  s : S
> SIZE / {} : S  =>  0 : S

ITER body: Apply the body expression to each element of a list. The body sequence has access to the stack.

:: (list 'elt) : 'A   ->  'A
     iff body :: [ 'elt : 'A -> 'A ]
> ITER body / { a ; <rest> } : S  =>  body ; ITER body / a : { <rest> } : S
> ITER body / {} : S  =>  S

Domain specific data types¶

timestamp: Dates in the real world.
mutez: A specific type for manipulating tokens.
contract 'param: A contract, with the type of its code.
address: An untyped contract address.
operation: An internal operation emitted by a contract.
key: A public cryptography key.
key_hash: The hash of a public cryptography key.
signature: A cryptographic signature.

Domain specific operations¶

Operations on timestamps¶

Current Timestamps can be obtained by the NOW operation, or retrieved from script parameters or globals.

ADD Increment / decrement a timestamp of the given number of seconds.

:: timestamp : int : 'S -> timestamp : 'S
:: int : timestamp : 'S -> timestamp : 'S

> ADD / seconds : nat (t) : S  =>  (seconds + t) : S
> ADD / nat (t) : seconds : S  =>  (t + seconds) : S

SUB Subtract a number of seconds from a timestamp.

:: timestamp : int : 'S -> timestamp : 'S

> SUB / seconds : nat (t) : S  =>  (seconds - t) : S

SUB Subtract two timestamps.

:: timestamp : timestamp : 'S -> int : 'S

> SUB / seconds(t1) : seconds(t2) : S  =>  (t1 - t2) : S

COMPARE: Timestamp comparison.

:: timestamp : timestamp : 'S   ->   int : 'S

> COMPARE / seconds(t1) : seconds(t2) : S  =>  -1 : S
    iff t1 < t2
> COMPARE / seconds(t1) : seconds(t2) : S  =>  0 : S
    iff t1 = t2
> COMPARE / seconds(t1) : seconds(t2) : S  =>  1 : S
    iff t1 > t2

Operations on Mutez¶

Mutez (micro-Tez) are internally represented by a 64 bit signed integers. There are restrictions to prevent creating a negative amount of mutez. Operations are limited to prevent overflow and mixing them with other numerical types by mistake. They are also mandatory checked for under/overflows.

ADD

:: mutez : mutez : 'S   ->   mutez : 'S

> ADD / x : y : S  =>  [FAILED]   on overflow
> ADD / x : y : S  =>  (x + y) : S

SUB

:: mutez : mutez : 'S   ->   mutez : 'S

> SUB / x : y : S  =>  [FAILED]
    iff   x < y
> SUB / x : y : S  =>  (x - y) : S

MUL

:: mutez : nat : 'S   ->   mutez : 'S
:: nat : mutez : 'S   ->   mutez : 'S

> MUL / x : y : S  =>  [FAILED]   on overflow
> MUL / x : y : S  =>  (x * y) : S

EDIV

:: mutez : nat : 'S   ->   option (pair mutez mutez) : 'S
:: mutez : mutez : 'S   ->   option (pair nat mutez) : 'S

> EDIV / x : 0 : S  =>  None
> EDIV / x : y : S  =>  Some (Pair (x / y) (x % y)) : S
    iff y <> 0

COMPARE

:: mutez : mutez : 'S -> int : 'S

> COMPARE / x : y : S  =>  -1 : S
    iff x < y
> COMPARE / x : y : S  =>  0 : S
    iff x = y
> COMPARE / x : y : S  =>  1 : S
    iff x > y

Operations on contracts¶

CREATE_CONTRACT { storage 'g ; parameter 'p ; code ... }: Forge a new contract from a literal.

:: key_hash : option key_hash : bool : bool : mutez : 'g : 'S
   -> operation : address : 'S

Originate a contract based on a literal. This is currently the only way to include transfers inside of an originated contract. The first parameters are the manager, optional delegate, then spendable and delegatable flags and finally the initial amount taken from the currently executed contract. The contract is returned as a first class value (to be dropped, passed as parameter or stored). The CONTRACT 'p instruction will fail until it is actually originated.

CREATE_ACCOUNT: Forge an account (a contract without code) creation operation.

:: key_hash : option key_hash : bool : mutez : 'S
   ->   operation : address : 'S

Take as argument the manager, optional delegate, the delegatable flag and finally the initial amount taken from the currently executed contract.

TRANSFER_TOKENS: Forge a transaction.

:: 'p : mutez : contract 'p : 'S   ->   operation : S

The parameter must be consistent with the one expected by the contract, unit for an account.

SET_DELEGATE: Forge a delegation.

:: option key_hash : 'S   ->   operation : S

BALANCE: Push the current amount of mutez of the current contract.

:: 'S   ->   mutez : 'S

ADDRESS: Push the address of a contract.

:: contract _ : 'S   ->   address : 'S

CONTRACT 'p: Push the untyped version of a contract.

:: address : 'S   ->   option (contract 'p) : 'S

> CONTRACT / addr : S  =>  Some addr : S
    iff addr exists and is a contract of parameter type 'p
> CONTRACT / addr : S  =>  Some addr : S
    iff 'p = unit and addr is an implicit contract
> CONTRACT / addr : S  =>  None : S
    otherwise

SOURCE: Push the contract that initiated the current transaction, i.e. the contract that paid the fees and storage cost, and whose manager signed the operation that was sent on the blockchain. Note that since TRANSFER_TOKENS instructions can be chained, SOURCE and SENDER are not necessarily the same.

:: 'S   ->   address : 'S

SENDER: Push the contract that initiated the current internal transaction. It may be the SOURCE, but may also not if the source sent an order to an intermediate smart contract, which then called the current contract.

:: 'S   ->   address : 'S

SELF: Push the current contract.

:: 'S   ->   contract 'p : 'S
   where   contract 'p is the type of the current contract

AMOUNT: Push the amount of the current transaction.

:: 'S   ->   mutez : 'S

IMPLICIT_ACCOUNT: Return a default contract with the given public/private key pair. Any funds deposited in this contract can immediately be spent by the holder of the private key. This contract cannot execute Michelson code and will always exist on the blockchain.

:: key_hash : 'S   ->   contract unit : 'S

GET_BALANCE: Returns the current balance of the contract in argument.

:: contract 'p : 'S   ->   mutez : 'S

IS_IMPLICIT: Returns the key_hash associated to the contract if it is an implicit one.

:: contract unit : 'S   ->   option key_hash : 'S

Special operations¶

STEPS_TO_QUOTA: Push the remaining steps before the contract execution must terminate.

:: 'S   ->   nat : 'S

NOW: Push the timestamp of the block whose validation triggered this execution (does not change during the execution of the contract).

:: 'S   ->   timestamp : 'S

BLOCK_LEVEL: Push the block level of the current block on the stack.

:: 'S   ->   nat : 'S

COLLECT_CALL: Push True on the stack if the current call is a collect call (where the contract pays fees).

:: 'S   ->   bool : 'S

Operations on bytes¶

Bytes are used for serializing data, in order to check signatures and compute hashes on them. They can also be used to incorporate data from the wild and untyped outside world.

PACK: Serializes a piece of data to its optimized binary representation.

:: 'a : 'S   ->   bytes : 'S

UNPACK 'a: Deserializes a piece of data, if valid.

:: bytes : 'S   ->   option 'a : 'S

CONCAT: Byte sequence concatenation.

:: bytes : bytes : 'S   -> bytes : 'S

 > CONCAT / s : t : S  =>  (s ^ t) : S

 :: bytes list : 'S   -> bytes : 'S

 > CONCAT / {} : S  =>  0x : S
 > CONCAT / { s ; <ss> } : S  =>  (s ^ r) : S
    where CONCAT / { <ss> } : S  =>  r : S

SIZE: size of a sequence of bytes.

:: bytes : 'S   ->   nat : 'S

SLICE: Bytes access.

:: nat : nat : bytes : 'S   -> option bytes : 'S

> SLICE / offset : length : s : S  =>  Some ss : S
   where ss is the substring of s at the given offset and of the given length
     iff offset and (offset + length) are in bounds
> SLICE / offset : length : s : S  =>  None : S
     iff offset or (offset + length) are out of bounds

COMPARE: Lexicographic comparison.

:: bytes : bytes : 'S   ->   int : 'S

> COMPARE / s : t : S  =>  -1 : S
    iff s < t
> COMPARE / s : t : S  =>  0 : S
    iff s = t
> COMPARE / s : t : S  =>  1 : S
    iff s > t

Cryptographic primitives¶

HASH_KEY: Compute the b58check of a public key.

:: key : 'S   ->   key_hash : 'S

BLAKE2B: Compute a cryptographic hash of the value contents using the Blake2B cryptographic hash function.

:: bytes : 'S   ->   bytes : 'S

SHA256: Compute a cryptographic hash of the value contents using the Sha256 cryptographic hash function.

:: bytes : 'S   ->   bytes : 'S

SHA512: Compute a cryptographic hash of the value contents using the Sha512 cryptographic hash function.

:: bytes : 'S   ->   bytes : 'S

CHECK_SIGNATURE: Check that a sequence of bytes has been signed with a given key.

:: key : signature : bytes : 'S   ->   bool : 'S

COMPARE:

:: key_hash : key_hash : 'S   ->   int : 'S

> COMPARE / x : y : S  =>  -1 : S
    iff x < y
> COMPARE / x : y : S  =>  0 : S
    iff x = y
> COMPARE / x : y : S  =>  1 : S
    iff x > y

Macros¶

In addition to the operations above, several extensions have been added to the language’s concrete syntax. If you are interacting with the node via RPC, bypassing the client, which expands away these macros, you will need to desugar them yourself.

These macros are designed to be unambiguous and reversible, meaning that errors are reported in terms of desugared syntax. Below you’ll see these macros defined in terms of other syntactic forms. That is how these macros are seen by the node.

Compare¶

Syntactic sugar exists for merging COMPARE and comparison combinators, and also for branching.

CMP{EQ|NEQ|LT|GT|LE|GE}

> CMP(\op) / S  =>  COMPARE ; (\op) / S

IF{EQ|NEQ|LT|GT|LE|GE} bt bf

> IF(\op) bt bf / S  =>  (\op) ; IF bt bf / S

IFCMP{EQ|NEQ|LT|GT|LE|GE} bt bf

> IFCMP(\op) / S  =>  COMPARE ; (\op) ; IF bt bf / S

Fail¶

The FAIL macros is equivalent to UNIT; FAILWITH and is callable in any context since it does not use its input stack.

FAIL

> FAIL / S  =>  UNIT; FAILWITH / S

Assertion macros¶

All assertion operations are syntactic sugar for conditionals with a FAIL instruction in the appropriate branch. When possible, use them to increase clarity about illegal states.

ASSERT

> ASSERT  =>  IF {} {FAIL}

ASSERT_{EQ|NEQ|LT|LE|GT|GE}

> ASSERT_(\op)  =>  IF(\op) {} {FAIL}

ASSERT_CMP{EQ|NEQ|LT|LE|GT|GE}

> ASSERT_CMP(\op)  =>  IFCMP(\op) {} {FAIL}

ASSERT_NONE

> ASSERT_NONE  =>  IF_NONE {} {FAIL}

ASSERT_SOME

> ASSERT_SOME @x =>  IF_NONE {FAIL} {RENAME @x}

ASSERT_LEFT

> ASSERT_LEFT @x =>  IF_LEFT {RENAME @x} {FAIL}

ASSERT_RIGHT

> ASSERT_RIGHT @x =>  IF_LEFT {FAIL} {RENAME @x}

Syntactic Conveniences¶

These are macros are simply more convenient syntax for various common operations.

DII+P code: A syntactic sugar for working deeper in the stack.

> DII(\rest=I*)P code / S  =>  DIP (DI(\rest)P code) / S

DUU+P: A syntactic sugar for duplicating the nth element of the stack.

> DUU(\rest=U*)P / S  =>  DIP (DU(\rest)P) ; SWAP / S

P(\left=A|P(\left)(\right))(\right=I|P(\left)(\right))R: A syntactic sugar for building nested pairs.

> PA(\right)R / S => DIP ((\right)R) ; PAIR / S
> P(\left)IR / S => PAIR ; (\left)R / S
> P(\left)(\right)R => (\right)R ; (\left)R ; PAIR / S

A good way to quickly figure which macro to use is to mentally parse the macro as P for pair constructor, A for left leaf and I for right leaf. The macro takes as many elements on the stack as there are leaves and constructs a nested pair with the shape given by its name.

Take the macro PAPPAIIR for instance:

P A  P P A  I    I R
( l, ( ( l, r ), r ))

A typing rule can be inferred:

PAPPAIIR
:: 'a : 'b : 'c : 'd : 'S  ->  (pair 'a (pair (pair 'b 'c) 'd))

UNP(\left=A|P(\left)(\right))(\right=I|P(\left)(\right))R: A syntactic sugar for destructing nested pairs. These macros follow the same convention as the previous one.

> UNPAIR / S => DUP ; CAR ; DIP { CDR } / S
> UNPA(\right)R / S => UNPAIR ; DIP (UN(\right)R) / S
> UNP(\left)IR / S => UNPAIR ; UN(\left)R / S
> UNP(\left)(\right)R => UNPAIR ; UN(\left)R ; UN(\right)R / S

C[AD]+R: A syntactic sugar for accessing fields in nested pairs.

> CA(\rest=[AD]+)R / S  =>  CAR ; C(\rest)R / S
> CD(\rest=[AD]+)R / S  =>  CDR ; C(\rest)R / S

IF_SOME bt bf: Inspect an optional value.

:: option 'a : 'S   ->   'b : 'S
   iff   bt :: [ 'a : 'S -> 'b : 'S]
         bf :: [ 'S -> 'b : 'S]

> IF_SOME / (Some a) : S  =>  bt / a : S
> IF_SOME / (None) : S  =>  bf / S

SET_CAR: Set the left field of a pair.

> SET_CAR  =>  CDR ; SWAP ; PAIR

SET_CDR: Set the right field of a pair.

> SET_CDR  =>  CAR ; PAIR

SET_C[AD]+R: A syntactic sugar for setting fields in nested pairs.

> SET_CA(\rest=[AD]+)R / S   =>
    { DUP ; DIP { CAR ; SET_C(\rest)R } ; CDR ; SWAP ; PAIR } / S
> SET_CD(\rest=[AD]+)R / S   =>
    { DUP ; DIP { CDR ; SET_C(\rest)R } ; CAR ; PAIR } / S

MAP_CAR code: Transform the left field of a pair.

> MAP_CAR code  =>  DUP ; CDR ; DIP { CAR ; code } ; SWAP ; PAIR

MAP_CDR code: Transform the right field of a pair.

> MAP_CDR code  =>  DUP ; CDR ; code ; SWAP ; CAR ; PAIR

MAP_C[AD]+R code: A syntactic sugar for transforming fields in nested pairs.

> MAP_CA(\rest=[AD]+)R code / S   =>
    { DUP ; DIP { CAR ; MAP_C(\rest)R code } ; CDR ; SWAP ; PAIR } / S
> MAP_CD(\rest=[AD]+)R code / S   =>
    { DUP ; DIP { CDR ; MAP_C(\rest)R code } ; CAR ; PAIR } / S

Concrete syntax¶

The concrete language is very close to the formal notation of the specification. Its structure is extremely simple: an expression in the language can only be one of the four following constructs.

An integer.
A character string.
The application of a primitive to a sequence of expressions.
A sequence of expressions.

This simple four cases notation is called Micheline.

The encoding of a Micheline source file must be UTF-8, and non-ASCII characters can only appear in comments and strings.

Constants¶

There are three kinds of constants:

Integers or naturals in decimal notation.
Strings, with usual escape sequences: \n, \t, \b, \r, \\, \". Unescaped line-breaks (both \n and \r) cannot appear in the middle of a string.
Byte sequences in hexadecimal notation, prefixed with 0x.

The current version of Michelson restricts strings to be the printable subset of 7-bit ASCII, plus the escaped characters mentioned above.

Primitive applications¶

A primitive application is a name followed by arguments

prim arg1 arg2

When a primitive application is the argument to another primitive application, it must be wrapped with parentheses.

prim (prim1 arg11 arg12) (prim2 arg21 arg22)

Sequences¶

Successive expression can be grouped as a single sequence expression using curly braces as delimiters and semicolon as separators.

{ expr1 ; expr2 ; expr3 ; expr4 }

A sequence can be passed as argument to a primitive.

prim arg1 arg2 { arg3_expr1 ; arg3_expr2 }

Primitive applications right inside a sequence cannot be wrapped.

{ (prim arg1 arg2) } # is not ok

Indentation¶

To remove ambiguities for human readers, the parser enforces some indentation rules.

For sequences:
- All expressions in a sequence must be aligned on the same column.
- An exception is made when consecutive expressions fit on the same line, as long as the first of them is correctly aligned.
- All expressions in a sequence must be indented to the right of the opening curly brace by at least one column.
- The closing curly brace cannot be on the left of the opening one.
For primitive applications:
- All arguments in an application must be aligned on the same column.
- An exception is made when consecutive arguments fit on the same line, as long as the first of them is correctly aligned.
- All arguments in a sequence must be indented to the right of the primitive name by at least one column.

Differences with the formal notation¶

The concrete syntax follows the same lexical conventions as the specification: instructions are represented by uppercase identifiers, type constructors by lowercase identifiers, and constant constructors are Capitalized.

All domain specific constants are Micheline constants with specific formats. Some have two variants accepted by the data type checker: a readable one in a string and an optimized.

mutez amounts are written as naturals.
timestamps are written either using RFC3339 notation in a string (readable), or as the number of seconds since Epoch in a natural (optimized).
contracts, addresses, keys and signatures are written as strings, in their usual Base58 encoded versions (readable), or as their raw bytes (optimized).

The optimized versions should not reach the RPCs, the protocol code will convert to optimized by itself when forging operations, storing to the database, and before hashing to get a canonical representation of a datum for a given type.

To prevent errors, control flow primitives that take instructions as parameters require sequences in the concrete syntax.

IF { instr1_true ; instr2_true ; ... }
   { instr1_false ; instr2_false ; ... }

Main program structure¶

The toplevel of a smart contract file must be an un-delimited sequence of three primitive applications (in no particular order) that provide its code, parameter and storage fields.

There can be an optional code @fee application for contracts which pay fees, see Contracts which pay fees.

See the section Examples for a concrete example.

Comments¶

A hash sign (#) anywhere outside of a string literal will make the rest of the line (and itself) completely ignored, as in the following example.

{ PUSH nat 1 ; # pushes 1
  PUSH nat 2 ; # pushes 2
  ADD }        # computes 2 + 1

Comments that span on multiple lines or that stop before the end of the line can also be written, using C-like delimiters (/* ... */).

Contracts which pay fees¶

Contract which include an additional code declared with code @fee, can pay fees for top-level callers of this contract. This code must return a single element on the stack of type

(pair mutez nat)

where the first component is the maximal amount of fees (in mutez) that the contract accepts to pay for its user and the second component is the maximal number of bytes in increased storage size for which the contract accepts to pay the burn.

The code is executed starting with a stack [ (Pair <parameter> <storage>) ], this way a contract can pay various amounts of fees depending on the call and its state.

When using the dune-client, a call to a contract that pays fees can be made with the option --collect-fee or --collect-fee-gas <limit> to specify the gas quantity that can be used to compute fees. Such calls will be rejected if their limit is above the hard limit or if they use more gas than the provided limit.

Calls which do not specify these options (i.e. for which the caller pays fees) do not trigger any additional code execution, and so can be made the same as before.

Annotations¶

The annotation mechanism of Michelson provides ways to better track data on the stack and to give additional type constraints. Annotations are only here to add constraints, i.e. they cannot turn an otherwise rejected program into an accepted one.

Stack visualization tools like the Michelson’s Emacs mode print annotations associated with each type in the program, as propagated by the typechecker as well as variable annotations on the types of elements in the stack. This is useful as a debugging aid.

We distinguish three kinds of annotations:

type annotations, written :type_annot,
variable annotations, written @var_annot,
and field or constructors annotations, written %field_annot.

Type annotations¶

Each type can be annotated with at most one type annotation. They are used to give names to types. For types to be equal, their unnamed version must be equal and their names must be the same or at least one type must be unnamed.

For instance, the following Michelson program which put its integer parameter in the storage is not well typed:

parameter (int :p) ;
storage (int :s) ;
code { UNPAIR ; SWAP ; DROP ; NIL operation ; PAIR }

Whereas this one is:

parameter (int :p) ;
storage int ;
code { UNPAIR ; SWAP ; DROP ; NIL operation ; PAIR }

Inner components of composed typed can also be named.

(pair :point (int :x_pos) (int :y_pos))

Push-like instructions, that act as constructors, can also be given a type annotation. The stack type will then have a correspondingly named type on top.

UNIT :t
:: 'A -> (unit :t) : 'A

PAIR :t
:: 'a : 'b : 'S -> (pair :t 'a 'b) : 'S

SOME :t
:: 'a : 'S -> (option :t 'a) : 'S

NONE :t 'a
:: 'S -> (option :t 'a) : 'S

LEFT :t 'b
:: 'a : 'S -> (or :t 'a 'b) : 'S

RIGHT :t 'a
:: 'b : 'S -> (or :t 'a 'b) : 'S

NIL :t 'a
:: 'S -> (list :t 'a) : 'S

EMPTY_SET :t 'elt
:: 'S -> (set :t 'elt) : 'S

EMPTY_MAP :t 'key 'val
:: 'S -> (map :t 'key 'val) : 'S

A no-op instruction CAST ensures the top of the stack has the specified type, and change its type if it is compatible. In particular, this allows to change or remove type names explicitly.

CAST 'b
:: 'a : 'S   ->   'b : 'S
   iff  'a = 'b

> CAST t / a : S  =>  a : S

Variable annotations¶

Variable annotations can only be used on instructions that produce elements on the stack. An instruction that produces n elements on the stack can be given at most n variable annotations.

The stack type contains both the types of each element in the stack, as well as an optional variable annotation for each element. In this sub-section we note:

[] for the empty stack,
@annot (top) : (rest) for the stack whose first value has type (top) and is annotated with variable annotation @annot and whose queue has stack type (rest).

The instructions which do not accept any variable annotations are:

DROP
SWAP
IF_NONE
IF_LEFT
IF_CONS
ITER
IF
LOOP
LOOP_LEFT
DIP
FAILWITH

The instructions which accept at most one variable annotation are:

DUP
PUSH
UNIT
SOME
NONE
PAIR
CAR
CDR
LEFT
RIGHT
NIL
CONS
SIZE
MAP
MEM
EMPTY_SET
EMPTY_MAP
UPDATE
GET
LAMBDA
EXEC
ADD
SUB
CONCAT
MUL
OR
AND
XOR
NOT
ABS
IS_NAT
INT
NEG
EDIV
LSL
LSR
COMPARE
EQ
NEQ
LT
GT
LE
GE
ADDRESS
CONTRACT
SET_DELEGATE
IMPLICIT_ACCOUNT
NOW
AMOUNT
BALANCE
HASH_KEY
CHECK_SIGNATURE
BLAKE2B
STEPS_TO_QUOTA
SOURCE
SENDER
SELF
CAST
RENAME

The instructions which accept at most two variable annotations are:

CREATE_ACCOUNT
CREATE_CONTRACT

Annotations on instructions that produce multiple elements on the stack will be used in order, where the first variable annotation is given to the top-most element on the resulting stack. Instructions that produce n elements on the stack but are given less than n variable annotations will see only their top-most stack type elements annotated.

CREATE_ACCOUNT @op @addr
:: key_hash : option key_hash : bool : mutez : 'S
   ->  @op operation : @addr address : 'S

CREATE_ACCOUNT @op
:: key_hash : option key_hash : bool : mutez : 'S
   ->  @op operation : address : 'S

A no-op instruction RENAME allows to rename variables in the stack or to erase variable annotations in the stack.

RENAME @new
:: @old 'a ; 'S -> @new 'a : 'S

RENAME
:: @old 'a ; 'S -> 'a : 'S

Field and constructor annotations¶

Components of pair types, option types and or types can be annotated with a field or constructor annotation. This feature is useful to encode records fields and constructors of sum types.

(pair :point
      (int %x)
      (int %y))

The previous Michelson type can be used as visual aid to represent the record type (given in OCaml-like syntax):

type point = { x : int ; y : int }

Similarly,

(or :t
    (int %A)
    (or
       (bool %B)
       (pair %C
             (nat %n1)
             (nat %n2))))

can be used to represent the algebraic data type (in OCaml-like syntax):

type t =
  | A of int
  | B of bool
  | C of { n1 : nat ; n2 : nat }

Field annotations are part of the type (at the same level as type name annotations), and so types with differing field names (if present) are not considered equal.

Instructions that construct elements of composed types can also be annotated with one or multiple field annotations (in addition to type and variable annotations).

PAIR %fst %snd
:: 'a : 'b : 'S -> (pair ('a %fst) ('b %snd)) : 'S

LEFT %left %right 'b
:: 'a : 'S -> (or ('a %left) ('b %right)) : 'S

RIGHT %left %right 'a
:: 'b : 'S -> (or ('a %left) ('b %right)) : 'S

NONE %some 'a
:: 'S -> (option ('a %some))

Some %some
:: 'a : 'S -> (option ('a %some))

To improve readability and robustness, instructions CAR and CDR accept one field annotation. For the contract to type check, the name of the accessed field in the destructed pair must match the one given here.

CAR %fst
:: (pair ('a %fst) 'b) : S -> 'a : 'S

CDR %snd
:: (pair 'a ('b %snd)) : S -> 'b : 'S

Syntax¶

Primitive applications can receive one or many annotations.

An annotation is a sequence of characters that matches the regular expression [@:%](|@|%|%%|[_a-zA-Z][_0-9a-zA-Z\.]*). They come after the primitive name and before its potential arguments.

(prim @v :t %x arg1 arg2 ...)

Ordering between different kinds of annotations is not significant, but ordering among annotations of the same kind is. Annotations of a same kind must be grouped together.

For instance these two annotated instructions are equivalent:

PAIR :t @my_pair %x %y

PAIR %x %y :t @my_pair

An annotation can be empty, in this case is will mean no annotation and can be used as a wildcard. For instance, it is useful to annotate only the right field of a pair instruction PAIR % %right or to ignore field access constraints, e.g. in the macro UNPPAIPAIR %x1 % %x3 %x4.

Annotations and macros¶

Macros also support annotations, which are propagated on their expanded forms. As with instructions, macros that produce n values on the stack accept n variable annotations.

DUU+P @annot
> DUU(\rest=U*)P @annot / S  =>  DIP (DU(\rest)P @annot) ; SWAP / S

C[AD]+R @annot %field_name
> CA(\rest=[AD]+)R @annot %field_name / S  =>  CAR ; C(\rest)R @annot %field_name / S
> CD(\rest=[AD]+)R @annot %field_name / S  =>  CDR ; C(\rest)R @annot %field_name / S

``CMP{EQ|NEQ|LT|GT|LE|GE}`` @annot
> CMP(\op) @annot / S  =>  COMPARE ; (\op) @annot / S

The variable annotation on SET_C[AD]+R and MAP_C[AD]+R annotates the resulting toplevel pair while its field annotation is used to check that the modified field is the expected one.

SET_C[AD]+R @var %field
> SET_CAR @var %field =>  CDR %field ; SWAP ; PAIR @var
> SET_CDR @var %field =>  CAR %field ; PAIR @var
> SET_CA(\rest=[AD]+)R @var %field / S   =>
  { DUP ; DIP { CAR ; SET_C(\rest)R %field } ; CDR ; SWAP ; PAIR @var } / S
> SET_CD(\rest=[AD]+)R  @var %field/ S   =>
  { DUP ; DIP { CDR ; SET_C(\rest)R %field } ; CAR ; PAIR @var } / S

MAP_C[AD]+R @var %field code
> MAP_CAR code  =>  DUP ; CDR ; DIP { CAR %field ; code } ; SWAP ; PAIR @var
> MAP_CDR code  =>  DUP ; CDR %field ; code ; SWAP ; CAR ; PAIR @var
> MAP_CA(\rest=[AD]+)R @var %field code / S   =>
  { DUP ; DIP { CAR ; MAP_C(\rest)R %field code } ; CDR ; SWAP ; PAIR @var} / S
> MAP_CD(\rest=[AD]+)R @var %field code / S   =>
 { DUP ; DIP { CDR ; MAP_C(\rest)R %field code } ; CAR ; PAIR @var} / S

Macros for nested PAIR and UNPAIR accept multiple annotations. Field annotations for PAIR give names to leaves of the constructed nested pair, in order. Variable annotations for UNPAIR give names to deconstructed components on the stack. This next snippet gives examples instead of generic rewrite rules for readability purposes.

PAPPAIIR @p %x1 %x2 %x3 %x4
:: 'a : 'b : 'c : 'd : 'S
   -> @p (pair ('a %x1) (pair (pair ('b %x) ('c %x3)) ('d %x4))) : 'S

PAPAIR @p %x1 %x2 %x3
:: 'a : 'b : 'c : 'S  ->  @p (pair ('a %x1) (pair ('b %x) ('c %x3))) : 'S

UNPAIR @x @y
:: (pair 'a 'b) : 'S -> @x 'a : @y 'b : 'S

UNPAPPAIIR @x1 @x2 @x3 @x4
:: (pair 'a (pair (pair 'b 'c) 'd )) : 'S
   -> @x1 'a : @x2 'b : @x3 'c : @x4 'd : 'S

Automatic variable and field annotations inferring¶

When no annotation is provided by the Michelson programmer, the typechecker infers some annotations in specific cases. This greatly helps users track information in the stack for bare contracts.

For unannotated accesses with CAR and CDR to fields that are named will be appended (with an additional . character) to the pair variable annotation.

CDAR
:: @p (pair ('a %foo) (pair %bar ('b %x) ('c %y))) : 'S ->  @p.bar.x 'b : 'S

If fields are not named but the pair is still named in the stack then .car or .cdr will be appended.

CDAR
:: @p (pair 'a (pair 'b 'c)) : 'S ->  @p.cdr.car 'b : 'S

If the original pair is not named in the stack, but a field annotation is present in the pair type the accessed value will be annotated with a variable annotation corresponding to the field annotation alone.

CDAR
:: (pair ('a %foo) (pair %bar ('b %x) ('c %y))) : 'S ->  @bar.x 'b : 'S

A similar mechanism is used for context dependent instructions:

ADDRESS  :: @c contract _ : 'S   ->   @c.address address : 'S

CONTRACT 'p  :: @a address : 'S   ->   @a.contract contract 'p : 'S

BALANCE :: 'S   ->   @balance mutez : 'S

SOURCE  :: 'S   ->   @source address : 'S

SENDER  :: 'S   ->   @sender address : 'S

SELF  :: 'S   ->   @self contract 'p : 'S

AMOUNT  :: 'S   ->   @amount mutez : 'S

STEPS_TO_QUOTA  :: 'S   ->  @steps nat : 'S

NOW  :: 'S   ->   @now timestamp : 'S

Inside nested code blocks, bound items on the stack will be given a default variable name annotation depending on the instruction and stack type (which can be changed). For instance the annotated typing rule for ITER on lists is:

ITER body
:: @l (list 'e) : 'A  ->  'A
   iff body :: [ @l.elt e' : 'A -> 'A ]

Special annotations¶

The special variable annotations @%% can be used on instructions CAR and CDR. It means to use the accessed field name (if any) as a name for the value on the stack. The following typing rule demonstrates their use for instruction CAR.

CAR @%
:: @p (pair ('a %fst) ('b %snd)) : 'S   ->   @fst 'a : 'S

CAR @%%
:: @p (pair ('a %fst) ('b %snd)) : 'S   ->   @p.fst 'a : 'S

The special variable annotation %@ can be used on instructions PAIR, SOME, LEFT, RIGHT. It means to use the variable name annotation in the stack as a field name for the constructed element. Two examples with PAIR follows, notice the special treatment of annotations with ..

PAIR %@ %@
:: @x 'a : @y 'b : 'S   ->   (pair ('a %x) ('b %y)) : 'S

PAIR %@ %@
:: @p.x 'a : @p.y 'b : 'S   ->  @p (pair ('a %x) ('b %y)) : 'S
:: @p.x 'a : @q.y 'b : 'S   ->  (pair ('a %x) ('b %y)) : 'S

JSON syntax¶

Micheline expressions are encoded in JSON like this:

An integer N is an object with a single field "int" whose value is the decimal representation as a string.

{ "int": "N" }
A string "contents" is an object with a single field "string" whose value is the decimal representation as a string.

{ "string": "contents" }
A sequence is a JSON array.

[ expr, ... ]
A primitive application is an object with two fields "prim" for

the primitive name and "args" for the arguments (that must contain an array). A third optional field "annots" contains a list of annotations, including their leading @, % or % sign.

{ "prim": "pair", "args": [ { "prim": "nat", "args": [] }, { "prim": "nat", "args": [] } ], "annots": [":t"] }

As in the concrete syntax, all domain specific constants are encoded as strings.

Examples¶

Contracts in the system are stored as a piece of code and a global data storage. The type of the global data of the storage is fixed for each contract at origination time. This is ensured statically by checking on origination that the code preserves the type of the global data. For this, the code of the contract is checked to be of type lambda (pair 'arg 'global) -> (pair (list operation) 'global) where 'global is the type of the original global store given on origination. The contract also takes a parameter and returns a list of internal operations, hence the complete calling convention above. The internal operations are queued for execution when the contract returns.

Empty contract¶

The simplest contract is the contract for which the parameter and storage are all of type unit. This contract is as follows:

code { CDR ;           # keep the storage
       NIL operation ; # return no internal operation
       PAIR };         # respect the calling convention
storage unit;
parameter unit;

Reservoir contract¶

We want to create a contract that stores tez until a timestamp T or a maximum amount N is reached. Whenever N is reached before T, all tokens are reversed to an account B (and the contract is automatically deleted). Any call to the contract’s code performed after T will otherwise transfer the tokens to another account A.

We want to build this contract in a reusable manner, so we do not hard-code the parameters. Instead, we assume that the global data of the contract are (Pair (Pair T N) (Pair A B)).

Hence, the global data of the contract has the following type

'g =
  pair
    (pair timestamp mutez)
    (pair (contract unit) (contract unit))

Following the contract calling convention, the code is a lambda of type

lambda
  (pair unit 'g)
  (pair (list operation) 'g)

written as

lambda
  (pair
     unit
     (pair
       (pair timestamp mutez)
       (pair (contract unit) (contract unit))))
  (pair
     (list operation)
     (pair
        (pair timestamp mutez)
        (pair (contract unit) (contract unit))))

The complete source reservoir.tz is:

parameter unit ;
storage
  (pair
     (pair (timestamp %T) (mutez %N)) # T N
     (pair (contract %A unit) (contract %B unit))) ; # A B
code
  { CDR ; DUP ; CAAR %T; # T
    NOW ; COMPARE ; LE ;
    IF { DUP ; CADR %N; # N
         BALANCE ;
         COMPARE ; LE ;
         IF { NIL operation ; PAIR }
            { DUP ; CDDR %B; # B
              BALANCE ; UNIT ;
              TRANSFER_TOKENS ;
              NIL operation ; SWAP ; CONS ;
              PAIR } }
       { DUP ; CDAR %A; # A
         BALANCE ;
         UNIT ;
         TRANSFER_TOKENS ;
         NIL operation ; SWAP ; CONS ;
         PAIR } }

Reservoir contract (variant with broker and status)¶

We basically want the same contract as the previous one, but instead of leaving it empty, we want to keep it alive, storing a flag S so that we can tell afterwards if the tokens have been transferred to A or B. We also want a broker X to get some fee P in any case.

We thus add variables P and S and X to the global data of the contract, now (Pair (S, Pair (T, Pair (Pair P N) (Pair X (Pair A B))))). P is the fee for broker A, S is the state, as a string "open", "timeout" or "success".

At the beginning of the transaction:

S is accessible via a CDAR
T               via a CDDAR
P               via a CDDDAAR
N               via a CDDDADR
X               via a CDDDDAR
A               via a CDDDDDAR
B               via a CDDDDDDR

The complete source scrutable_reservoir.tz is:

parameter unit ;
storage
  (pair
     string # S
     (pair
        timestamp # T
        (pair
           (pair mutez mutez) # P N
           (pair
              (contract unit) # X
              (pair (contract unit) (contract unit)))))) ; # A B
code
  { DUP ; CDAR ; # S
    PUSH string "open" ;
    COMPARE ; NEQ ;
    IF { FAIL } # on "success", "timeout" or a bad init value
       { DUP ; CDDAR ; # T
         NOW ;
         COMPARE ; LT ;
         IF { # Before timeout
              # We compute (P + N) mutez
              PUSH mutez 0 ;
              DIP { DUP ; CDDDAAR } ; ADD ; # P
              DIP { DUP ; CDDDADR } ; ADD ; # N
              # We compare to the cumulated amount
              BALANCE ;
              COMPARE; LT ;
              IF { # Not enough cash, we just accept the transaction
                   # and leave the global untouched
                   CDR ; NIL operation ; PAIR }
                 { # Enough cash, successful ending
                   # We update the global
                   CDDR ; PUSH string "success" ; PAIR ;
                   # We transfer the fee to the broker
                   DUP ; CDDAAR ; # P
                   DIP { DUP ; CDDDAR } ; # X
                   UNIT ; TRANSFER_TOKENS ;
                   # We transfer the rest to A
                   DIP { DUP ; CDDADR ; # N
                         DIP { DUP ; CDDDDAR } ; # A
                         UNIT ; TRANSFER_TOKENS } ;
                   NIL operation ; SWAP ; CONS ; SWAP ; CONS ;
                   PAIR } }
            { # After timeout, we refund
              # We update the global
              CDDR ; PUSH string "timeout" ; PAIR ;
              # We try to transfer the fee to the broker
              BALANCE ; # available
              DIP { DUP ; CDDAAR } ; # P
              COMPARE ; LT ; # available < P
              IF { BALANCE ; # available
                   DIP { DUP ; CDDDAR } ; # X
                   UNIT ; TRANSFER_TOKENS }
                 { DUP ; CDDAAR ; # P
                   DIP { DUP ; CDDDAR } ; # X
                   UNIT ; TRANSFER_TOKENS } ;
              # We transfer the rest to B
              DIP { BALANCE ; # available
                    DIP { DUP ; CDDDDDR } ; # B
                    UNIT ; TRANSFER_TOKENS } ;
              NIL operation ; SWAP ; CONS ; SWAP ; CONS ;
              PAIR } } }

Forward contract¶

We want to write a forward contract on dried peas. The contract takes as global data the tons of peas Q, the expected delivery date T, the contract agreement date Z, a strike K, a collateral C per ton of dried peas, and the accounts of the buyer B, the seller S and the warehouse W.

These parameters as grouped in the global storage as follows:

Pair
  (Pair (Pair Q (Pair T Z)))
  (Pair
     (Pair K C)
     (Pair (Pair B S) W))

of type

pair
  (pair nat (pair timestamp timestamp))
  (pair
     (pair mutez mutez)
     (pair (pair account account) account))

The 24 hours after timestamp Z are for the buyer and seller to store their collateral (Q * C). For this, the contract takes a string as parameter, matching "buyer" or "seller" indicating the party for which the tokens are transferred. At the end of this day, each of them can send a transaction to send its tokens back. For this, we need to store who already paid and how much, as a (pair mutez mutez) where the left component is the buyer and the right one the seller.

After the first day, nothing can happen until T.

During the 24 hours after T, the buyer must pay (Q * K) to the contract, minus the amount already sent.

After this day, if the buyer didn’t pay enough then any transaction will send all the tokens to the seller.

Otherwise, the seller must deliver at least Q tons of dried peas to the warehouse, in the next 24 hours. When the amount is equal to or exceeds Q, all the tokens are transferred to the seller. For storing the quantity of peas already delivered, we add a counter of type nat in the global storage. For knowing this quantity, we accept messages from W with a partial amount of delivered peas as argument.

After this day, any transaction will send all the tokens to the buyer (not enough peas have been delivered in time).

Hence, the global storage is a pair, with the counters on the left, and the constant parameters on the right, initially as follows.

Pair
  (Pair 0 (Pair 0_00 0_00))
  (Pair
     (Pair (Pair Q (Pair T Z)))
     (Pair
        (Pair K C)
        (Pair (Pair B S) W)))