#13823: Use NonEmpty lists in more places in the GHC API -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: task | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.3 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: #8782 Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- After GHC 8.2 is released and we drop support for building GHC with 7.10, we should try to use `Data.List.NonEmpty` in more places in the GHC API. I ran into this issue recently when using some functions from `ListSetOps`: {{{#!hs removeDups :: (a -> a -> Ordering) -> [a] -> ([a], [[a]]) findDupsEq :: (a -> a -> Bool) -> [a] -> [[a]] equivClasses :: (a -> a -> Ordering) -> [a] -> [[a]] }}} These type signatures are terrible. Really, they should be: {{{#!hs removeDups :: (a -> a -> Ordering) -> [a] -> ([a], [NonEmpty a]) findDupsEq :: (a -> a -> Bool) -> [a] -> [NonEmpty a] equivClasses :: (a -> a -> Ordering) -> [a] -> [NonEmpty a] }}} Since 90% of the time, the first thing you do after finding duplicates is to take a representative from the duplicate set. With lists, this requires the partial operation `head`, but with `NonEmpty`, this can be total like it was intended to be. I'm sure there are other places in the API that could benefit from `NonEmpty`, so if you have any suggestions, please leave them here. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13823 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler