Typed holes are fantastic already, but could be even better. Consider the following three examples:

Example 1

f :: Eq a => a -> a -> Bool
f x y = _

GHC-7.8.2 shows:

    Found hole ‘_’ with type: Bool
    Relevant bindings include
      y :: a (bound at THC.hs:4:5)
      x :: a (bound at THC.hs:4:3)
      f :: a -> a -> Bool (bound at THC.hs:4:1)
    In the expression: _
    In an equation for ‘f’: f x y = _

GHC additionally knows that Eq a holds. It would be great if it would print that, too.

Example 2

g :: (forall a. Eq a => a -> Bool) -> Bool
g f = f _ 

GHC-7.8.2 shows:

    Found hole ‘_’ with type: a0
    Where: ‘a0’ is an ambiguous type variable
    Relevant bindings include
      f :: forall a. Eq a => a -> Bool (bound at THC.hs:7:3)
      g :: (forall a. Eq a => a -> Bool) -> Bool (bound at THC.hs:7:1)
    In the first argument of ‘f’, namely ‘_’
    In the expression: f _
    In an equation for ‘g’: g f = f _

GHC additionally knows the Eq a0 must hold. It would be nice if it could indicate that.

Example 3

data X a where
  Y :: X Int

h :: X a -> a -> a
h Y x = _

GHC-7.8.2 shows:

    Found hole ‘_’ with type: Int
    Relevant bindings include
      x :: a (bound at THC.hs:13:5)
      h :: X a -> a -> a (bound at THC.hs:13:1)
    In the expression: _
    In an equation for ‘h’: h Y x = _

Interestingly, GHC shows that we have to produce an Int. Unfortunately, it shows that x is of type a. GHC additionally knows that a ~ Int. It would be nice if GHC could either apply the constraint to show that x :: Int or at least show the constraint.

Trac metadata

Trac field	Value
Version	7.8.2
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture