list comprehension

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list comprehension

(functional programming)
An expression in a functional language 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 <>, (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 "ZF expressions", 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].
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In that region the generator expression, can be linearized in each operating point.
Among specific suggestions are consider generator expressions for large comprehensions, know how closures interact with variable scope, prefer public attributes over private ones, use plain attributes instead of get and set methods, use queue to coordinate work between threads, use built-in algorithms and data structures, know how to break circular dependencies, and profile before optimizing.