Parallel List comprehensions

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Brief Explanation

Parallel comprehensions extend list comprehensions with a notation for zips. The comprehension

[ e | quals1 | ... | qualsN ]

can be desugared to

zipWithN (\ p1 ... pN -> e) [p1 | quals1] ... [pN | qualsN]

where pi is a tuple of the variables defined by qualsi and used by e.



add Parallel List comprehensions


  • Easy and well-specified.
  • Expresses zips of filters, which are tricky to express with standard list comprehensions.
  • Those who use them do so heavily, e.g. there are 142 uses of parallel list comprehensions in the jhc source tree.


  • Some people do not find them useful, and find the only slightly longer version using explicit zip clearer.
  • If we went back to monad comprehensions at some point, parallel comprehensions would not make sense.
  • One more concept to learn (and implement), with relatively low payoff.
  • Cannot express filters of zips. If you want to filter the list returned by a parallel list comprehension, you have to go back to using the zip form, or separately filter the result.
  • Might cause confusing errors if an extra | is typed by accident.
  • GHC has a more general mechanism in the form of Comprehensive Comprehensions (paper, wiki page). Unfortunately GHC's implementation does not currently have the feature enabled due to the difficulty in finding syntax for it that is parsable (see the section on "Bracketing syntax" in the wiki page above).
Last modified 11 years ago Last modified on Mar 27, 2008 9:39:21 PM