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

signature REF =sigdatatyperef =datatype refval !        : 'a ref -> 'aval :=       : 'a ref * 'a -> unitval exchange : 'a ref * 'a -> 'aval swap     : 'a ref * 'a ref -> unitval app    : ('a -> unit) -> 'a ref -> unitval map    : ('a -> 'b) -> 'a ref -> 'b refval modify : ('a -> 'a) -> 'a ref -> unitendstructure Ref : REF

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

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

• r := a
  Makes the referencer refer to valuea.This function is the same as the global:= operator and is also part of theGeneral structure.

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

• swap (r1, r2)
  Swaps the values referred to by the referencesr1 andr2.

• app f r
  Applies the functionf to the value referred to by the referencer. Equivalent tof (!r).

• map f r
  Creates a new reference that refers to the valuef a, wherea is the value referred to to byr. Equivalent toref (f (!r)).

• modify f r
  Makes the referencer refer to the valuef a, wherea is the value previously referred to.This expression is equivalent tor := f (!r).

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 theGeneral structure for backward compatibility.

The addition of this module does not affect existing programs.

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

• [2015-08-16] Proposed