Strange error message with Idris interfaces

Question

I am trying to implement a hierarchy of simple algebraic structures using Idris interfaces. The code is as follows:

module AlgebraicStructures

-- definition of some algebraic structures in terms of type classes

%access public export

Associative : {a : Type} -> (a -> a -> a) -> Type
Associative {a} op = (x : a) -> 
                     (y : a) ->
                     (z : a) ->
                     (op x (op y z)) = (op (op x y) z)

Identity : {a : Type} -> (a -> a -> a) -> a -> Type
Identity op v = ((x : a) -> (op x v) = x, 
                 (x : a) -> (op v x) = x)                 


Commutative : {a : Type} -> (a -> a -> a) -> Type
Commutative {a} op = (x : a) ->
                     (y : a) ->
                     (op x y) = (op y x)


infixl 4 <**>

interface IsMonoid a where
  empty  : a
  (<**>) : a -> a -> a 
  assoc  : Associative (<**>) 
  ident  : Identity (<**>) empty


interface IsMonoid a => IsCommutativeMonoid a where
  comm : Commutative (<**>)

But Idris gives this strange error message:

When checking type of constructor of AlgebraicStructures.IsCommutativeMonoid:
Can't find implementation for IsMonoid a

I believe Idris interfaces work like classes like Haskell. In Haskell, it should work. Am I doing something stupid?

Answer 1

, , , -, a Commutative (<**>), , <**> . a, , - Commutative {a} (<**>) - , , a .