Question

In my very simple boolean expression toy program, I have the following evaluation function:

eval' :: Expr -> M.Map Char Bool -> Bool

eval' (Const c) values = c

eval' (Var v)   values = M.findWithDefault False v values

eval' (Not x)   values = not (eval' x values)

eval' (And a b) values = eval' a values && eval' b values

eval' (Or  a b) values = eval' a values || eval' b values

eval' (Xor a b) values = eval' a values /= eval' b values

I was wondering if there was a way to pass the values table implicitly? Maybe with the help of Monads?

Answer 1

In this case, do not pass values at all:

eval' :: Expr -> M.Map Char Bool -> Bool
eval' e values = eval'' e

    where
    eval'' (Const c) = c
    eval'' (Var v) = M.findWithDefault False v values
    eval'' (Not x) = not (eval'' x)
    ...