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Local exceptions/labels/goto/return.

This module defines a mechanism akin to SML's exception generators or to a generalization of C's return, i.e. the ability to define local labels, which may be used for immediately terminating an expression and returning a value. By opposition to usual OCaml exceptions, this mechanism

  • allows polymorphic return values
  • makes accidental exception catching slightly harder (while a local exception can escape its scope, it cannot be caught again by accident from this module).

Example:

let find_in_array a e =
  label (fun label ->
    for i = 0 to Array.length a - 1 do
      if Array.get a i = e then return label (Some i)
    done;
    None)
  • author David Teller

@documents Return

type 'a t

A label which may be used to return values of type 'a

val label : ('a t -> 'a) -> 'a

label f creates a new label x and invokes f x. If, during the execution of f, return x v is invoked, the execution of f x stops immediately and label f returns v. Otherwise, if f x terminates normally and returns y, label f returns y.

Calling return x v from outside scope f is a run-time error and causes termination of the program.

val with_label : ('a t -> 'a) -> 'a

as label

val return : 'a t -> 'a -> _

Return to a label. return l v returns to the point where label l was obtained and produces value l.

Calling return l v from outside the scope of l (i.e. the call to function label which produced l) is a run-time error and causes termination of the program.