Question

I am getting this error, and I don’t know what I am doing wrong.
I am new to Haskell, so please elaborate everything for me.

import Data.Maybe

data Op = Add | Sub | Mul | Div | And | Or | Not | Eq | Less | Great
    deriving (Eq, Show)

data Exp = Literal Value
     | Primitive Op [Exp]
     | Variable String
     | If Exp Exp Exp
     | Let [(String, Exp)] Exp
    deriving (Show, Eq)

data Value = Number Int
       | Bool Bool
       | String String
    deriving (Eq, Show)

type Env = [(String, Value)]

eval :: Env -> Exp -> Value
eval e (Literal v) = v
eval e (Variable x) = fromJust (lookup x e)   --22

prim :: Op -> [Value] -> Value
prim Add [Number a, Number b] = Number (a + b)
prim And [Bool a, Bool b] = Bool (a && b)
prim Sub [Number a, Number b] = Number (a - b)
prim Mul [Number a, Number b] = Number (a * b)
prim Div [Number a, Number b] = Number (a `div` b)
prim Or [Bool a, Bool b] = Bool (a || b)
prim Not [Bool a] = Bool (not a)
prim Eq [Number a, Number b] = Bool (a == b)
prim Eq [String a, String b] = Bool (a == b) 
prim Less [Number a, Number b] = Bool (a < b)
prim Less [String a, String b] = Bool (a < b)
prim Great [Number a, Number b] = Bool (a > b)
prim Great [String a, String b] = Bool (a > b) --37

main = do
    n = "n"    -- parse error on input `='
    nv = Variable "n"   -- parse error on input `='
    lit n = Literal (Number n)  

    t0 = Primitive Mul [lit 5, lit 2]    
    t1 = Let [(n, t0)] (If (Primitive Great [nv, lit 9]) (lit 1) (lit 0))  -- parse error on input `='

I wrote in comments where I am getting the errors.
I checked the validness of the parameters, and everything looks fine. It’s maybe an error with my syntax… I guess.

Thank you in advance.

Answer 1

Your main function is broken in almost every way. Have you learned monads or for this case only about the IO monad?

main :: IO Exp
main = do   
  let n = "n"
      nv = Variable "n"
      lit n = Literal (Number n)
      t0 = Primitive Mul [lit 5, lit 2]
      in return $ Let [(n, t0)] (If (Primitive Great [nv, lit 9]) (lit 1) (lit 0))

You can’t just use = in the context of that do-Block.
This version works as intended. Learn about the let expression and the return. If you don’t know this, read for example here.