Haskell: parse error on input `='

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.

Please don't edit the question to remove the parts that the answers depend on to make sense. — Abizern
– Abizern, Commented Feb 4, 2013 at 7:53

Secoe · Accepted Answer · 2013-02-02 02:19:02Z

4

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.

edited Feb 2, 2013 at 2:19

answered Feb 2, 2013 at 2:02

Secoe

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6 Comments

Nayana Over a year ago

Thank you for the help. It now complies, but it doesn't compute. It's supposed to give me a value.

Nayana Over a year ago

In my main, I am trying this "eval [("y", (Number 40))] (Let [("x", (Literal (Number 2)))] (Primitive Add [(Variable "x"), (Variable "y")]))" which is supposed to 42. But it gives me this error "Couldn't match expected type IO t0' with actual type Value'" Do you know what I might be missing?

Secoe Over a year ago

You didn't tell it to do. You initial code does not contain any reference to your eval function.

Secoe Over a year ago

You have to feed the result of eval to return. Like return $ eval .... Your code above will btw still not work because your eval function is incomplete.

is7s Over a year ago

There is no for the in in the last line.

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