Re: [Haskell-beginners] Problems with Data types a la carte

Am Freitag 23 April 2010 22:52:19 schrieb Rodrigo Geraldo:

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:

Your problem is the wrong associativity, the type signature is parsed as Expr ((Val :+: Add) :+: Mult) while you need Expr (Val :+: (Add :+: Mult)) Insert the parentheses or add a fixity declaration infixr k :+: to Expr.hs (where k must be replaced with an integer from 0 through 9).

Regards

Rodrigo