[Haskell-cafe] Which type variables are allowed in a context?

Vladimir Reshetnikov v.reshetnikov at gmail.com
Sun May 31 03:24:06 EDT 2009


Consider this (a bit pathological) Haskell code:

class A a where
  foo :: A (b d) => a (c b)

GHC compiles it successfully, but Hugs rejects it:

Ambiguous type signature in class declaration
*** ambiguous type : (A a, A (b c)) => a (d b)
*** assigned to    : foo

What is the correct behavior, and which part of the haskell 98 report
explains this?


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