On 2015-07-30 at 21:39, Michael Karg
I guess without that pragma, the string literals already imply t ~ [] for Foldable t.
You're basically right. Without OverloadedStrings, a string literal ("foo") is always a String. With OverloadedStrings, it's free to be any t such that IsString t. (But it's expected to have a monomorphic type, not the polymorphic "foo" :: forall t. IsString t => t) Hence the second error below - GHC tries to pick a type that is both Foldable and IsString, but that's not a unique combination.
GHC 7.10 fails with the following errors (whereas 7.8 compiles without complaining):
ghc --make "Testcase.hs" [1 of 1] Compiling Main ( Testcase.hs, Testcase.o ) Testcase.hs:7:31: No instance for (Foldable t0) arising from a use of ‘elem’ The type variable ‘t0’ is ambiguous (...)
Testcase.hs:8:15: No instance for (Data.String.IsString (t0 Char)) arising from the literal ‘"$_-"’ The type variable ‘t0’ is ambiguous (...)
Question for GHC devs: How hard would it be to give a different error message instead of "No instance ..." when the type variable is ambiguous? I always find this error slightly misleading, since it seems to me that there are multiple valid instances, not that there is "no instance". bergey