[Haskell-cafe] Re: Syntax question: class (Monad m) => MonadReader r m | m -> r where

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It's called a "functional dependency". This is not part of the Haskell-98 language standard; check the GHC manual.

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The documentation says "There should be more documentation, but there isn't (yet). Yell if you need it." :)

you probably don't found it. since 6.6 (or 6.8?) ghc includes great tutorial on fundeps derived from hugs docs

It's here, on 'latest' directory: http://www.haskell.org/ghc/docs/latest/html/users_guide/type-class-extension... But yes, you're right, there's a lot of good examples and explanations. But that comment is still there, just in the end of the first paragraph. Best, Maurício