Batteries user guide : BatReturn

# Module BatReturn

    module BatReturn: sig .. end

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(s): David Teller

    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 -> 'b

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.