Version 7 (modified by simonmar@…, 8 years ago) (diff)

rewrite the cons section

Parallel List comprehensions

See ExtensionDescriptionHowto for information on how to write these extension descriptions. Please add any new extensions to the list of HaskellExtensions.

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.
  • 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