wiki:ParallelListComp

Version 3 (modified by ross@…, 8 years ago) (diff)

summarize mailing list discussion

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.

References

Tickets

#55
add Parallel List comprehensions

Pros

  • Easy and well-specified

Cons

  • Not widely used, and not hugely more concise than using explicit zips.
  • Naive users can trip over them if they misplace a '|'.
  • In its more general forms it's hard to predict how the elements from each set of generators and filters will match up, e.g.
    [(i,j,k) | i <- [1..3], j <- [1..3] | k <- [1..9]]
    
  • The notation expresses zips of filters and maps, but not filters of zips, which are more common.