Haskell Syntax: Parse Error On Input

Question

As part of a mini-haskell compiler that I'm writing, I have a function named app. What I want this function to do is take in these arguments epp (App e1 e2). The first step would be to evaluate e1 recursively (epp e1) and check if the output would be an error. If not then evaluate e2 and then call another function eppVals to evaluate the outputs of the calls on e1 and e2 which I defined as v1 and v2 respectively.

Answer 1

In your hpaste you have a function appVals which has been renamed to eppVals in your question above. This is unhelpful.

Let's look at some types:

• epp :: Exp -> Error Val
• appVals :: Val -> Val -> Error Val

The error message Couldn't match expected type Val with actual type Error Val is trying to tell you that the first parameter to appVals is expected to have type Val (see type signature for appVals) but the actual type of the value you've provided as the first parameter is v1, defined as epp e1, which has type Error Val (see type signature for epp).

Val and Error Val are not the same type. That's your problem.

How to fix it? You need some way of handling the error cases separately from the rest of the computation. If you have implemented the Applicative or Monad classes for your Error type, this is almost certainly what they do.

Edit: You don't include the definition of your Error, but I expect it looks like this:

data Error a = S a | Error String

Implement a couple of classes for it (you'll need to import Control.Applicative):

instance Functor Error where
    fmap f (S x)     = S (f x)
    fmap _ (Error s) = Error s

instance Applicative Error where
    pure = S
    Error s <*> _       = Error s
    _       <*> Error s = Error s
    S f     <*> S x     = S (f x)

Now you can rewrite

epp (App e1 e2) = eppVals <$> epp e1 <*> epp e2