#8761: Make pattern synonyms work with Template Haskell -------------------------------------+------------------------------------- Reporter: goldfire | Owner: Type: feature request | Status: new Priority: normal | Milestone: 8.0.1 Component: Template Haskell | Version: Resolution: | Keywords: | PatternSynonyms 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 sboo): one motivation is for fixed points https://www.reddit.com/r/haskell/comments/3qrkzr/one_weird_trick_for_nicer_f... for example: {{{#!hs {-# LANGUAGE TemplateHaskell, PatternSynonyms, DeriveFunctor #-} data TreeF a r = LitF a | VarF String | BinF r Operation r deriving Functor data Operation = Add | ... makeFix ''TreeF }}} would generate: {{{#!hs type Tree a = Fix (TreeF a) pattern Lit :: a -> Tree a pattern Lit a = Fix (LitF a) pattern Var :: String -> Tree a pattern Var s = Fix (VarF s) pattern Bin :: Tree a -> Operation -> Tree a -> Tree a pattern Bin l op r = Fix (BinF l op r) }}} which would make `TreeF` as easy to use as `Tree` when using pattern matching: {{{#!hs optimize :: Tree a -> Tree a optimize (Bin (Lit x) Add (Lit y)) = Lit (x + y) optimize ... = ... }}} and still as easy when using recursion schemes: {{{#!hs evalTreeF :: Map String Integer -> TreeF Integer (Maybe Integer) -> Maybe Integer evalTreeF environment (BinF (Just l) Add (Just r)) = Just (l + r) evalTreeF ... evalTree :: Map String Integer -> Tree Integer -> Maybe Integer evalTree environment = cata (evalTreeF environment) }}} (sorry if here's the wrong place, this is my first post) -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8761#comment:11 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler