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 p_i is a tuple of the variables defined by quals_i and used by e.

References

•  Parallel list comprehensions in the GHC User's Guide

Tickets

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add Parallel List comprehensions

Pros

• Easy and well-specified
• Expresses zips of filters, which are tricky to express with standard list comprehensions which are more suited towards filters of zips.

Cons

• Not widely used, and not hugely more concise than using explicit zips. (this is disputable, there are 142 uses of parallel list comprehensions in the jhc source tree for instance)
• 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]]