#12001: RFC: Add pattern synonyms to base -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: Type: feature request | Status: new Priority: normal | Milestone: Component: libraries/base | Version: 7.10.3 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: | -------------------------------------+------------------------------------- Comment (by Iceland_jack): For [https://hackage.haskell.org/package/base-4.9.0.0/docs/Data-List- NonEmpty.html Data.List.NonEmpty] {{{#!hs pattern NonEmpty :: NonEmpty a -> [a] pattern NonEmpty xs <- (nonEmpty -> Just xs) where NonEmpty xs = toList xs {-# COMPLETE [], NonEmpty #-} }}} using PatternSynonyms/CompleteSigs. == Uses == The pattern of destructing a `NonEmpty` value only to reassemble it pop up when using `NonEmpty` {{{#!hs f [] = ... f (x:xs) = ... (x:|xs) }}} An example is {{{#!hs -- match_groups :: [[(PatGroup,EquationInfo)]] -> DsM (NonEmpty MatchResult) -- match_groups [] = matchEmpty v ty -- match_groups (g:gs) = mapM match_group (g:|gs) match_groups :: [[(PatGroup,EquationInfo)]] -> DsM (NonEmpty MatchResult) match_groups [] = matchEmpty v ty match_groups (NonEmpty gs) = mapM match_group gs }}} Which can be abstracted into this rather useful eliminator: {{{#!hs -- match_groups = neElim (matchEmpty v ty) (mapM match_group) neElim :: c -> (NonEmpty a -> c) -> [a] -> c neElim c _ [] = c neElim _ f (NonEmpty xs) = f xs }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12001#comment:11 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler