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It seems to me that every instance of the token pattern in the attached code sample should be highlighted with the keyword face (i.e.: the same face as module, class, where, etc.).
The main objection to this kind of fix is that people could want to name a function pattern when the extension is disabled. However, the proc keyword has similar problems but is currently being syntax-highlighted regardless of the presence of the relevant language pragma. I guess proc is technically in the Haskell 2010 Report so it's Official Haskell whereas -XPatternSynonyms is merely GHC Haskell, but I think that doesn't hold much water given the popularity of GHC extensions.
{-# LANGUAGE GADTs #-}
{-# LANGUAGE PatternSynonyms #-}
{-# LANGUAGE UnicodeSyntax #-}
{-# LANGUAGE ViewPatterns #-}
-- Pattern synonyms can be exported
module Main (pattern A1) where
data Foo = Foo
data Bar where Bar :: (Show a) => a -> Bar
-- Basic usage of unidirectional and bidirectional pattern synonyms
pattern A1 <- Foo
pattern A2 = Foo
-- Patterns can have type signatures
-- These type signatures have up to two contexts
-- (these are called CProv and CReq for proven and required constraints resp.)
pattern B1 :: (Show a) => (Show a) => a -> Bar
pattern B1 x <- Bar x
pattern B2 :: (Show a) => (Show a) => a -> Bar
pattern B2 x = Bar x
-- If CProv is empty, it can be omitted.
pattern C1 :: (Show a) => a -> Bar
pattern C1 x <- Bar x
pattern C2 :: (Show a) => a -> Bar
pattern C2 x = Bar x
-- If CProv is not empty but CReq is, it's denoted like so:
pattern D1 :: (Show a) => () => a -> Bar
pattern D1 x <- Bar x
pattern D2 :: (Show a) => () => a -> Bar
pattern D2 x = Bar x
-- Explicitly-bidirectional pattern synonym
pattern Succ n <- (subtract 1 -> n) where Succ n = n + 1
-- With -XUnicodeSyntax, ← can be used for a unidirectional synonym definition
pattern Baz ← Foo
-- For comparison, a function definition:
foo :: Int -> Int
foo x = x + 5
main :: IO ()
main = return ()Metadata
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