Related question probably: why ghc compiles this:
{-# LANGUAGE RankNTypes, ImpredicativeTypes #-} methods :: [(String, forall b. Eq b => b)] methods = [ ("method1", undefined ) , ("method2", undefined) ]
test:: [String] test= pmap methods where pmap = map fst
But when I change 'test' to:
test:: [String] test= map fst methods
I get: Cannot match a monotype with `forall b. (Eq b) => b' Expected type: [(String, b)] Inferred type: [(String, forall b1. (Eq b1) => b1)] In the second argument of `map', namely `methods' In the expression: map fst methods Failed, modules loaded: none. -- View this message in context: http://old.nabble.com/type-class-constraints-headache-tp27752745p27779518.ht... Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.