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