#14374: group by using groupWith seems stricter than it need be -------------------------------------+------------------------------------- Reporter: newthin | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Description changed by newthin: Old description:
Using TransformListComps:
{{{#!hs import GHC.Exts (the, groupWith)
is = [1..10] fs = map ((,) False) is ts = map ((,) True ) is
a = [(the b, take 5 n) | (b,n) <- ts ++ fs, then group by b using groupWith] b = [(the b, take 5 n) | (b,n) <- ts ++ fs ++ [undefined], then group by b using groupWith] c = take 2 b }}}
As of GHC 8.2.1, both b and c fail.
While getting b to work seems difficult - it would require realizing that the domain of Bool has been satisfied - c seems like it might be more feasible to get working.
As a motivator, this pattern is pretty common in data science, where we want to map elements of a large list to a small result domain and get examples of those elements with each result without iterating through the whole list.
New description: Using TransformListComps: {{{#!hs import GHC.Exts (the, groupWith) is = [1..10] fs = map ((,) False) is ts = map ((,) True ) is r1 = [(the b, take 5 n) | (b,n) <- ts ++ fs, then group by b using groupWith] r2 = [(the b, take 5 n) | (b,n) <- ts ++ fs ++ [undefined], then group by b using groupWith] r3 = take 2 r2 }}} As of GHC 8.2.1, both r2 and r3 fail. While getting r2 to work seems difficult - it would require realizing that the domain of Bool has been satisfied - r3 seems like it might be more feasible to get working. As a motivator, this pattern is pretty common in data science, where we want to map elements of a large list to a small result domain and get examples of those elements with each result without iterating through the whole list. -- -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14374#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler