Monad comprehensions, group by
Summary
I was reading https://gitlab.haskell.org/ghc/ghc/-/wikis/monad-comprehensions.
What happened to group by
?
Steps to reproduce
box% ghci
GHCi, version 8.10.2: https://www.haskell.org/ghc/ :? for help
Prelude> :set -XMonadComprehensions -XTransformListComp -XParallelListComp
Prelude> [(x :: [Int]) | x <- [1,2,1,2], then group by x ] :: [[Int]]
<interactive>:2:49: error: parse error on input ‘]’
Prelude>
Expected behavior
Documentation says
[(x :: [Int]) | x <- [1,2,1,2], then group by x ] :: [[Int]]
-- translates to:
do x <- mgroupWith (\x -> x) [1,2,1,2]
return x
but I can't find mgroupWith
.
But this works
Prelude GHC.Exts> [ x | x <- [1,2,1,2], then group by x using GHC.Exts.groupWith]
[[1,1],[2,2]]
Prelude GHC.Exts> GHC.Exts.groupWith id [1,2,1,2]
[[1,1],[2,2]]
So change the documentation?
Environment
- GHC version used: GHCi 8.10.2
Optional:
- Operating System: Arch Linux
- System Architecture: 64-bit PC