allow more type signatures

(idea triggered by #393 (closed))

Allow multiple copies of a type-signature in a module, such that it is an error if they're not equivalent, but they don't have to be syntactically equal ( f :: ShowS f :: String -> String is okay).

It would also be nice to allow any name in scope at top-level (even if imported) to be given a type signature. But that raises a question: can these type signatures give a more specific type than that of the raw imported function, the way normal function type signatures can with regards to their implementation?

Use cases: (1. making sure multiple implementations give the same interface, generally varying by #ifdef) (2. asserting that something's type can be specified in two different weird ways). I don't really want to abandon having a type-signature right above every function definition even if it is a duplicate.

(1.) would be fixed by allowing type signatures in export lists instead. I suppose these could be more restrictive than in the module and not affect the module, e.g.

module X (idN :: Int -> Int, true) where
idN n = n
true :: Bool
true = idN True

Trac metadata

Trac field	Value
Version	6.6.1
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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Operating system	Unknown
Architecture	Unknown