I am confused about the behaviour of the AllowAmbiguousTypes extension. I tried the basic example of the user manual:
class C a where f :: C a => Int f = 3 main = print f
Using ghc 7.8.3, without the extension, this will cause a type error at the type signature (line 2). Adding AllowAmbgiuousTypes causes an error at the call site instead (line 4). So far so good. However, if I replace the (C a) constraint with (Num a), no type error is reported, and the example runs fine! This is true with or without the extension. Using ghc 7.6.3, however, the example is still rejected with an "ambiguous constraint" error. Is this a bug in ghc 7.8? Is ghc somehow able to prove the Num constraint even though a is universally quantified? -- Audun Skaugen