2015 007 Addition of Ref module

Proposal 2015-007

Addition of Ref module

Author: Andreas Rossberg
Last revised: August 16, 2015
Status: proposed
Discussion: issue #8

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Synopsis

signature REF =
  sig
    datatype ref = datatype ref

    val !        : 'a ref -> 'a
    val :=       : 'a ref * 'a -> unit
    val exchange : 'a ref * 'a -> 'a
    val swap     : 'a ref * 'a ref -> unit

    val app    : ('a -> unit) -> 'a ref -> unit
    val map    : ('a -> 'b) -> 'a ref -> 'b ref
    val modify : ('a -> 'a) -> 'a ref -> unit
  end

structure Ref : REF

Description

• datatype 'a ref
  The type of mutable references (same as the global type ref).

• ! r
  Returns the value referred to by r. This function is the same as the global ! operator and is also part of the General structure.

• r := a
  Makes the reference r refer to value a. This function is the same as the global := operator and is also part of the General structure.

• exchange (r, a)
  Makes the reference r refer to value a and returns the previously referenced value in a single operation.

• swap (r1, r2)
  Swaps the values referred to by the references r1 and r2.

• app f r
  Applies the function f to the value referred to by the reference r. Equivalent to f (!r).

• map f r
  Creates a new reference that refers to the value f a, where a is the value referred to to by r. Equivalent to ref (f (!r)).

• modify f r
  Makes the reference r refer to the value f a, where a is the value previously referred to. This expression is equivalent to r := f (!r).

Rationale

This module provides a few combinators that are sometimes useful when applying higher-order functions to data structures containing references.

It also provides a natural home for the existing toplevel operators, but they would also remain in the General structure for backward compatibility.

Impact

The addition of this module does not affect existing programs.

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History

• [2015-08-22] Added some text about the operators that are also in the General structure. [JHR].

• [2015-08-16] Proposed

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