I’m trying to define a function folde of type folde :: (Int -> a) -> (a -> a -> a) -> Expr -> a such that it replaces each Val constructor in an expression with the function f, and each Add constructor with the function g. The data type I’m using is data Expr = Val Int | Add Expr Expr.

I’ve written the following code:

data Expr = Val Int | Add Expr Expr
folde :: (Int -> a) -> (a -> a -> a) -> Expr -> a
folde f g (Val n) = f n
folde f g (Add z x) = g z x

However, I received the error Couldn't match expected type 'a' with actual type 'Expr', when I run the code. Could you please help me identify and clarify the error?

The error occurs because the return type of the folde function is specified as a, but the second argument g expects two values of the same type a. However, in the case of Add z x, the two arguments z and x have the type Expr, which doesn’t match the expected type a.

To fix this issue, you need to recursively apply the folde function to each subexpression z and x before applying the function g. Here’s the corrected code:

data Expr = Val Int | Add Expr Expr
folde :: (Int -> a) -> (a -> a -> a) -> Expr -> a
folde f g (Val n) = f n
folde f g (Add z x) = g (folde f g z) (folde f g x)

This should resolve the error and allow the function to compile successfully.