On 2015-05-18 at 11:29:25 +0200, Simon Peyton Jones wrote:
Have you tested this? If GHC sees two overlapping instances instance ... => IsString [a] instance IsString [Char] it’ll refrain from using the former until it knows that the latter can’t match.
FWIW, the original stated problem of having GHC be able to infer "hi" ++ "bye" under the influence of -XOverloadedStrings which would cause GHC to complain λ:2> "foo" ++ "bar" <interactive>:2:1: Non type-variable argument in the constraint: Data.String.IsString [a] (Use FlexibleContexts to permit this) When checking that ‘it’ has the inferred type it :: forall a. Data.String.IsString [a] => [a] is actually resolved by the following patch (which I tried w/ GHC HEAD): --8<---------------cut here---------------start------------->8--- diff --git a/libraries/base/Data/String.hs b/libraries/base/Data/String.hs index a03569f..2bed477 100644 --- a/libraries/base/Data/String.hs +++ b/libraries/base/Data/String.hs @@ -1,5 +1,5 @@ {-# LANGUAGE Trustworthy #-} -{-# LANGUAGE NoImplicitPrelude, FlexibleInstances #-} +{-# LANGUAGE NoImplicitPrelude, GADTs #-} ----------------------------------------------------------------------------- -- | @@ -34,6 +34,6 @@ import Data.List (lines, words, unlines, unwords) class IsString a where fromString :: String -> a -instance IsString [Char] where +instance (a ~ Char) => IsString [a] where fromString xs = xs --8<---------------cut here---------------end--------------->8---