I don't understand the difference
forall a isx. (Is isx a, Show a) => isx -> String and (Is isx a, Show a) => isx -> String I have used the `forall` explicitly only to fix the order of the type
On 11/02/2017 09:50, Nicholls, Mark wrote: parameters (`a` and `isx`) so that we are sure to set the type of `a` when we write (using TypeApplications): foo4 @Y (X,Y) In the second declaration, the `forall` is implicit.
I think my understanding of type classes is naïve, I just thought it meant that secretly a dictionary was being passed. Yes your understanding is correct. The issue here is that the compiler doesn't know the type of `a`, hence it can't select and pass the appropriate instances.
the compiler would identify the specific dictionary from the call site Even at call site, the compiler can't infer the `a` type from the `isx` type (nor from the return type of `foo4`).
Do you want the `a` type to be dependent on the `isx` type? I.e., to only be allowed to define a single `Is isx a` instance for each `isx` type. -- Sylvain