Conflicting definitions in pattern synonyms using a parameter more than once

Question

Why can't I define this pattern synonym?

pattern Double a = (a,a)

error:
    • Conflicting definitions for ‘a’
      Bound at: <interactive>:1:21
                <interactive>:1:23
    • In a pattern synonym declaration

Answer 1

This could only work by equality-comparing the two elements of the tuple, but patterns don't just automatically spawn Eq constraints for this purpose.

You can get this functionality by using a view pattern in the definition:

{-# LANGUAGE PatternSynonyms, ViewPatterns #-}

fromRedunTuple :: Eq a => (a,a) -> Maybe a
fromRedunTuple (x,y)
 | x==y       = Just x
 | otherwise  = Nothing

pattern Double a <- (fromRedunTuple -> Just a)
 where Double a = (a,a)