#10794: Extension request: "where" clauses in type signatures -------------------------------------+------------------------------------- Reporter: danso | Owner: Type: feature | Status: new request | Priority: lowest | Milestone: Component: Compiler | Version: 7.10.2 (Type checker) | Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Revisions: | -------------------------------------+------------------------------------- Some type signatures could be simplified by separating certain elements, either because they are lengthy and can be expressed simpler, to better express intention, or because they are repetitive. A "WhereSignatures" extension would allow for type signatures like the following: {{{#!hs fold :: f -> [a] -> a where f :: a -> a -> a -- expresses intention more clearly than: fold :: (a -> a -> a) -> [a] -> a union :: a -> a -> a where a :: Map.Map k v -- much shorter/more readable than: union :: Map.Map k v -> Map.Map k v -> Map.Map k v }}} Or maybe even: {{{#!hs fmap :: (a -> b) -> f a -> f b where (Functor f) -- arguably prettier than: fmap :: (Functor f) => (a -> b) -> f a -> f b }}} This minor syntactic sugar is essentially equivalent to local "type" definitions, which provides an advantage of making the type's purpose more clear. This seems like a natural development on what the type checker can do. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10794 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler