list comprehension

functional programming An expression in a functionallanguage denoting the results of some operation on (selected)

elements of one or more lists. An example in Haskell:

[ (x,y) | x #@- [1 .. 6], y #@- [1 .. x], x+y #@ 10]

This returns all pairs of numbers (x,y) where x and y are

elements of the list 1, 2, ..., 10, y = x and their sum is

less than 10.

A list comprehension is simply "syntactic sugar" for a

combination of applications of the functions, concat, map and

filter. For instance the above example could be written:

filter p (concat (map ( x - map ( y -@# (x,y))

[1..x]) [1..6]))

where

p (x,y) = x+y #@ 10

According to a note by Rishiyur Nikhil [email protected],

(August 1992), the term itself seems to have been coined by

Phil Wadler circa 1983-5, although the programming construct

itself goes back much further (most likely Jack Schwartz and

the SETL language).

The term "list comprehension" appears in the references below.

The earliest reference to the notation is in Rod Burstall and

John Darlington's description of their language, NPL.

David Turner subsequently adopted this notation in his

languages SASL, KRC and Miranda, where he has called them "ZFexpressions", set abstractions and list abstractions (in his

1985 FPCA paper [Miranda: A Non-Strict Functional Language

with Polymorphic Types]).

["The OL Manual" Philip Wadler, Quentin Miller and Martin

Raskovsky, probably 1983-1985].

["How to Replace Failure by a List of Successes" FPCA

September 1985, Nancy, France, pp. 113-146].

(1995-02-22)