Hi !

I've tried to implement the expression interpreter present in Data types a la carte, putting the new "cases" of my data type in a separate module. When I'm trying to create a simple
expression like this:

x :: Expr (Val :+: Add :+: Mult)
x = (val 2000 `mult` val 399) `plus` (val 3)

I'm getting the following type error that I can't solve:

src/Main.hs:25:4:
    No instance for (Add :<: Mult)
      arising from a use of `plus' at src/Main.hs:25:4-43
    Possible fix: add an instance declaration for (Add :<: Mult)
    In the expression: (val 2000 `mult` val 399) `plus` (val 3)
    In the definition of `x':
        x = (val 2000 `mult` val 399) `plus` (val 3)

src/Main.hs:25:5:
    No instance for (Val :<: Mult)
      arising from a use of `val' at src/Main.hs:25:5-12
    Possible fix: add an instance declaration for (Val :<: Mult)
    In the first argument of `mult', namely `val 2000'
    In the first argument of `plus', namely `(val 2000 `mult` val 399)'
    In the expression: (val 2000 `mult` val 399) `plus` (val 3)

I've attached the source code...

Regards

Rodrigo