Make HsIf more polymorphic with RebindableSyntax

Currently, ifThenElse is typed this way:

ifThenElse :: pred_ty -> b_ty -> b_ty -> res_ty

I propose to change this to:

ifThenElse :: pred_ty -> b1_ty -> b2_ty -> res_ty

to support expressions like this:

ifThenElse :: Maybe a -> (a -> b) -> b -> b
ifThenElse (Just a) f _ = f a
ifThenElse Nothing  _ e = e

example :: String
example = if Just 2
             then \v -> show v
             else "No value"

The fix is very simple and just one more line to add to the typechecker. Patch is attached.

Trac metadata

Trac field	Value
Version	7.0.1
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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