Functions
LIGO functions are the basic building block of contracts. For example, entrypoints are functions and each smart contract needs a main function that dispatches control to the entrypoints (it is not already the default entrypoint).
The semantics of function calls in LIGO is that of a copy of the arguments but also of the environment. In the case of JsLIGO, this means that any mutation (assignment) on variables outside the scope of the function will be lost when the function returns, just as the mutations inside the functions will be.
Declaring Functions
Functions in CameLIGO are defined using the let keyword, like other
values. The difference is that a succession of parameters is provided
after the value name, followed by the return type. This follows OCaml
syntax. For example:
let add (a : int) (b : int) : int = a + b
You can call the function add defined above using the LIGO compiler
like this:
ligo run evaluate-expr \
gitlab-pages/docs/language-basics/src/functions/add.mligo \
'add 1 2'
# Outputs: 3
CameLIGO is a little different from other syntaxes when it comes to function parameters. In OCaml, functions can only take one parameter. To get functions with multiple arguments like we are used to in imperative programming languages, a technique called currying is used. Currying essentially translates a function with multiple arguments into a series of single argument functions, each returning a new function accepting the next argument until every parameter is filled. This is useful because it means that CameLIGO supports partial application.
Currying is however not the preferred way to pass function arguments in CameLIGO. While this approach is faithful to the original OCaml, it is costlier in Michelson than naive function execution accepting multiple arguments. Instead, for most functions with more than one parameter, we should gather the arguments in a tuple and pass the tuple in as a single parameter.
Here is how you define a basic function that accepts two integers and returns an integer as well:
let add (a, b : int * int) : int = a + b // Uncurried
let add_curry (a : int) (b : int) : int = add (a, b) // Curried
let increment : int -> int = add_curry 1 // Partial application
You can run the increment function defined above using the LIGO
compiler like this:
ligo run evaluate-call \
gitlab-pages/docs/language-basics/src/functions/curry.mligo \
increment 5
# Outputs: 6
The function body is a single expression, whose value is returned.
By default, LIGO will warn about unused arguments inside
functions. In case we do not use an argument, we can use the wildcard
_ to prevent warnings. Either use _ instead of the argument
identifier:
let k (x : int) (_ : int) = x
or use an identifier starting with wildcard:
let k (x : int) (_y : int) = x
Sometimes, one has to chain multiple function applications. In this case, parentheses are needed.
let f (x : int) = x + 1
let g (x : int) = x - 2
let h (x : int) = x + x - 3
(* Here we apply function f on value 42,
then apply g on the result,
and then apply h on the result *)
let result = h (g (f 42))
(* Parentheses are indeed necessary here. If we remove them, we have : *)
// let result' = h g f 42
(* Which is different, it is equivalent to : *)
// let result' = ((h g) f) 42
Here, one might want to reduce the number of parentheses, for readability.
In this case, the reverse-application operator (|>) can be used instead.
Expression f x can be rewritten as x |> f,
and g (f x) can be rewritten as x |> f |> g
(you can think of it as "I take x, give it to function f, and then the result to function g").
Above result can thus be rewritten as :
let result = 42 |> f |> g |> h
Function application has precedence over reverse-application operator,
which means f 42 |> g is the same as (f 42) |> g and not f (42 |> g).
So you can rewrite result as :
let result = f 42 |> g |> h
This can be useful when you have to deal with a long chain of function calls.
This operator actually comes from OCaml's pervasives. Other similar operators will be added when enabling support for custom operator definition.
Functions in JsLIGO can be defined in two main ways: using the keyword
function or const (the keyword let is defaulted to const in
this instance). The latter manner is preferred when the function body
is an expression. For example, here is how you define a basic function
that sums two integers:
const add = (a: int, b: int) => a + b;
You can call the function add defined above using the LIGO compiler
like this:
ligo run evaluate-expr \
gitlab-pages/docs/language-basics/src/functions/add.jsligo \
'add(1,2)'
# Outputs: 3
If the body contains statements instead of a single expression, you
would use a block and a return statement:
const myFun = (x: int, y: int) => {
const doubleX = x + x;
const doubleY = y + y;
return doubleX + doubleY;
};
although it is arguably more readable to use function, like so:
function myFun2 (x: int, y: int) {
const doubleX = x + x;
const doubleY = y + y;
return doubleX + doubleY;
}
Note that JsLIGO, like JavaScript, requires the return keyword to indicate
what is being returned. If return is not used, it will be the same as
return unit.
By default, LIGO will warn about unused arguments inside
functions. In case we do not use an argument, its name should start with
_ to prevent warnings.
const k_other = (x: int, _y: int) => x;
Anonymous functions (a.k.a. lambdas)
It is possible to define functions without assigning them a name. They are useful when you want to pass them as arguments, or assign them to a key in a record or a map.
Here is how to define an anonymous function:
let increment (b : int) : int = (fun (a : int) -> a + 1) b
let a = increment 1 // a = 2
You can check the value of a defined above using the LIGO compiler
like this:
ligo run evaluate-expr gitlab-pages/docs/language-basics/src/functions/anon.mligo a
# Outputs: 2
const increment = (b) => ((a) => a + 1) (b);
const a = increment(1); // a == 2
You can check the value of a defined above using the LIGO compiler
like this:
ligo run evaluate-expr gitlab-pages/docs/language-basics/src/functions/anon.jsligo a
# Outputs: 2
If the example above seems contrived, here is a more common design pattern for lambdas: to be used as parameters to functions. Consider the use case of having a list of integers and mapping the increment function to all its elements.
let incr_map (l : int list) : int list =
List.map (fun (i : int) -> i + 1) l
You can call the function incr_map defined above using the LIGO compiler
like so:
ligo run evaluate-call \
gitlab-pages/docs/language-basics/src/functions/incr_map.mligo \
incr_map "[1;2;3]"
# Outputs: CONS(2 , CONS(3 , CONS(4 , LIST_EMPTY()))), equivalent to [ 2 ; 3 ; 4 ]
let incr_map = l => List.map(i => i + 1, l);
You can call the function incr_map defined above using the LIGO compiler
like so:
ligo run evaluate-call \
gitlab-pages/docs/language-basics/src/functions/incr_map.jsligo \
incr_map "list([1,2,3])"
# Outputs: CONS(2 , CONS(3 , CONS(4 , LIST_EMPTY()))), equivalent to list([ 2 , 3 , 4 ])
Nested functions (also known as closures)
It is possible to define a functions inside another function. These functions have access to variables in the same scope.
let closure_example (i : int) : int =
let closure = fun (j : int) -> i + j in
closure i
function closure_example (i) {
let closure = j => i + j;
return closure(i);
};
Recursive functions
In CameLIGO, recursive functions are defined using the rec keyword
let rec sum (n, acc : int * int) : int =
if n < 1 then acc else sum (n-1, acc + n)
let rec fibo (n, n_1, n_0 : int * int * int) : int =
if n < 2 then n_1 else fibo (n-1, n_1 + n_0, n_1)
In JsLigo, recursive functions are defined and called using the same syntax as non-recursive functions.
function sum (n: int, acc: int): int {
if (n < 1) return acc else return sum(n-1, acc + n);
};
function fibo (n: int, n_1: int, n_0: int): int {
if (n < 2) return n_1 else return fibo (n-1, n_1 + n_0, n_1);
};