Haskell TypeApplication-like way of explicitly picking an implementation of an interface

Question

I'm using Idris 2 v0.3. I can't see why the compiler can't find an implementation for Gaussian of Distribution in

interface Distribution dist where
  mean : dist -> Double

data Gaussian : Type where
  MkGaussian : Double -> Gaussian

Distribution Gaussian where
    mean (MkGaussian mean') = mean'

KnowledgeBased : Distribution d => (d : Type) -> Type

ei : KnowledgeBased Gaussian

I'm getting error

"src/Foo.idr" 12L, 459C written
Error: While processing type of ei. Can't find an implementation for Distribution ?d.

.../src/Foo.idr:12:6--12:29
    |
 12 | ei : KnowledgeBased Gaussian
    |      ^^^^^^^^^^^^^^^^^^^^^^^

Answer 1

Your scoping is wrong; in Distribution d => (d : Type) -> Type, the Distribution d part implicitly binds d, and then it is shadowed by d : Type.

Instead what you want is:

KnowledgeBased : (d : Type) -> Distribution d => Type

Here, the first d : Type binds d with a Pi; it parses as (d : Type) -> (Distribution d => Type). So the d in Distribution d is now bound, and no automatic implicit binding is done.

However, this is not idiomatic Idris. Instead, you should use the Distribution d-bound implicit parameter and not worry about tricks to make it explicit:

KnowledgeBased : Distribution d => Type

ei : KnowledgeBased {d = Gaussian}