Re: [Haskell-cafe] GHCi infers a type but refuses it as type signature

Simple: the definition of MonadState uses those extensions.

Thanks, yes it helps and explains all. :^) I suppose then that if -XFlexibleContexts is indeed required by the standard libraries, it is a "safe" extension, meaning supported by all compilers? Are many such extensions de-facto standard that anyone can enable by default?