An alternative to witness values

So, I might have come up with a nicer notation to providing type witnesses.

Say I have a class thusly:

class Metric a where
    measure :: Vector f -> Vector f -> f

and an example instance:

instance Metric "Euclidean" where
    measure = sum . (zipWith (*))

In order to actually use a specific metric, one would have to modify the type signature to include a witness, and use it such as measure undefined::"Euclidean" a b. Personally, I find such expressions to be ugly clutter.

Instead, here are three alternate proposals which don't require including a witness:

1. (Metric "Euclidean") => <term involving measure> for example dot a b = (Metric "Euclidean") => measure a b
2. New keyword similar to "where"

 dot a b = measure a b assuming (Metric "Euclidean")

1. New keyword similar to "let..in"

dot a b = assume (Metric "Euclidean") in measure a b

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Trac field	Value
Version	7.9
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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