ligo/gitlab-pages/docs/language-basics/loops.md

id	title
loops	Loops

General Iteration

General iteration in PascaLIGO takes the shape of general loops, which should be familiar to programmers of imperative languages as "while loops". Those loops are of the form while <condition> <block>. Their associated block is repeatedly evaluated until the condition becomes true, or never evaluated if the condition is false at the start. The loop never terminates if the condition never becomes true. Because we are writing smart contracts on Tezos, when the condition of a "while" loops fails to become true, the execution will run out of gas and stop with a failure anyway.

Here is how to compute the greatest common divisors of two natural numbers by means of Euclid's algorithm:

function gcd (var x : nat; var y : nat) : nat is
  block {
    if x < y then {
      const z : nat = x;
      x := y; y := z
    }
    else skip;
    var r : nat := 0n;
    while y =/= 0n block {
      r := x mod y;
      x := y;
      y := r
    }
  } with x

You can call the function gcd defined above using the LIGO compiler like so:

ligo run-function
gitlab-pages/docs/language-basics/src/loops/gcd.ligo gcd '(2n*2n*3n*11n, 2n*2n*2n*3n*3n*5n*7n)'
# Outputs: +12

CameLIGO is a functional language where user-defined values are constant, therefore it makes no sense in CameLIGO to feature loops, which we understand as syntactic constructs where the state of a stopping condition is mutated, as with "while" loops in PascaLIGO.

Instead, CameLIGO implements a folded operation by means of a predefined function named Loop.fold_while. It takes an initial value of a certain type, called an accumulator, and repeatedly calls a given function, called folded function, that takes that accumulator and returns the next value of the accumulator, until a condition is met and the fold stops with the final value of the accumulator. The iterated function needs to have a special type: if the type of the accumulator is t, then it must have the type bool * t (not simply t). It is the boolean value that denotes whether the stopping condition has been reached.

Here is how to compute the greatest common divisors of two natural numbers by means of Euclid's algorithm:

let iter (x,y : nat * nat) : bool * (nat * nat) =
  if y = 0n then false, (x,y) else true, (y, x mod y)

let gcd (x,y : nat * nat) : nat =
  let x,y = if x < y then y,x else x,y in
  let x,y = Loop.fold_while iter (x,y)
  in x

To ease the writing and reading of the iterated functions (here, iter), two predefined functions are provided: continue and stop:

let iter (x,y : nat * nat) : bool * (nat * nat) =
  if y = 0n then stop (x,y) else continue (y, x mod y)

let gcd (x,y : nat * nat) : nat =
  let x,y = if x < y then y,x else x,y in
  let x,y = Loop.fold_while iter (x,y)
  in x

You can call the function gcd defined above using the LIGO compiler like so:

ligo run-function
gitlab-pages/docs/language-basics/src/loops/gcd.mligo gcd (2n*2n*3n*11n, 2n*2n*2n*3n*3n*5n*7n)'
# Outputs: +12

ReasonLIGO is a functional language where user-defined values are constant, therefore it makes no sense in ReasonLIGO to feature loops, which we understand as syntactic constructs where the state of a stopping condition is mutated, as with "while" loops in PascaLIGO.

Instead, ReasonLIGO features a fold operation as a predefined function named Loop.fold_while. It takes an initial value of a certain type, called an accumulator, and repeatedly calls a given function, called iterated function, that takes that accumulator and returns the next value of the accumulator, until a condition is met and the fold stops with the final value of the accumulator. The iterated function needs to have a special type: if the type of the accumulator is t, then it must have the type bool * t (not simply t). It is the boolean value that denotes whether the stopping condition has been reached.

Here is how to compute the greatest common divisors of two natural numbers by means of Euclid's algorithm:

let iter = ((x,y) : (nat, nat)) : (bool, (nat, nat)) =>
  if (y == 0n) { (false, (x,y)); } else { (true, (y, x mod y)); };

let gcd = ((x,y) : (nat, nat)) : nat => {
  let (x,y) = if (x < y) { (y,x); } else { (x,y); };
  let (x,y) = Loop.fold_while (iter, (x,y));
  x
};

To ease the writing and reading of the iterated functions (here, iter), two predefined functions are provided: continue and stop:

let iter = ((x,y) : (nat, nat)) : (bool, (nat, nat)) =>
  if (y == 0n) { stop ((x,y)); } else { continue ((y, x mod y)); };

let gcd = ((x,y) : (nat, nat)) : nat => {
  let (x,y) = if (x < y) { (y,x); } else { (x,y); };
  let (x,y) = Loop.fold_while (iter, (x,y));
  x
};

Bounded Loops

In addition to general loops, PascaLIGO features a specialised kind of loop to iterate over bounded intervals. These loops are familiarly known as "for loops" and they have the form for <variable assignment> to <upper bound> <block>, as found in imperative languages.

Consider how to sum the natural numbers up to n:

function sum (var n : nat) : int is block {
  var acc : int := 0;
  for i := 1 to int (n) block {
    acc := acc + i
  }
} with acc

(Please do not use that function: there exists a closed form formula.)

You can call the function sum defined above using the LIGO compiler like so:

ligo run-function
gitlab-pages/docs/language-basics/src/loops/sum.ligo sum 7n
# Outputs: 28

PascaLIGO "for" loops can also iterate through the contents of a collection, that is, a list, a set or a map. This is done with a loop of the form for <element var> in <collection type> <collection var> <block>, where <collection type> is any of the following keywords: list, set or map.

Here is an example where the integers in a list are summed up.

function sum_list (var l : list (int)) : int is block {
  var total : int := 0;
  for i in list l block {
    total := total + i
  }
} with total

You can call the function sum_list defined above using the LIGO compiler like so:

ligo run-function
gitlab-pages/docs/language-basics/src/loops/collection.ligo sum_list
'list [1;2;3]'
# Outputs: 6

Here is an example where the integers in a set are summed up.

function sum_set (var s : set (int)) : int is block {
  var total : int := 0;
  for i in set s block {
    total := total + i
  }
} with total

You can call the function sum_set defined above using the LIGO compiler like so:

ligo run-function
gitlab-pages/docs/language-basics/src/loops/collection.ligo sum_set
'set [1;2;3]'
# Outputs: 6

Loops over maps are actually loops over the bindings of the map, that is, a pair key-value noted key -> value (or any other variables). Given a map from strings to integers, here is how to sum all the integers and concatenate all the strings.

Here is an example where the keys are concatenated and the values are summed up.

function sum_map (var m : map (string, int)) : string * int is block {
  var string_total : string := "";
  var int_total : int := 0;
  for key -> value in map m block {
    string_total := string_total ^ key;
    int_total := int_total + value
  }
} with (string_total, int_total)

You can call the function sum_map defined above using the LIGO compiler like so:

ligo run-function
gitlab-pages/docs/language-basics/src/loops/collection.ligo sum_map
'map ["1"->1; "2"->2; "3"->3]'
# Outputs: ( "123", 6 )