Allright, so I have put all of the example code for Calculator in chapter 4 on page ~140 in a file and when loading it into GHCI it throws this:
λ: :reload
[1 of 1] Compiling Calculator ( Calculator.hs, interpreted )
Calculator.hs:40:9: error:
• Couldn't match expected type ‘String -> Either String a1’
with actual type ‘Either String b0’
• In the pattern: Left err
In a case alternative: Left err -> Left err
In the expression:
case readEither of
Left err -> Left err
Right lit' -> Right (Lit lit', rest)
|
40 | Left err -> Left err
| ^^^^^^^^
Calculator.hs:41:9: error:
• Couldn't match expected type ‘String -> Either String a1’
with actual type ‘Either a0 Int’
• In the pattern: Right lit'
In a case alternative: Right lit' -> Right (Lit lit', rest)
In the expression:
case readEither of
Left err -> Left err
Right lit' -> Right (Lit lit', rest)
|
41 | Right lit' -> Right (Lit lit', rest)
| ^^^^^^^^^^
Failed, no modules loaded.
I am not sure if maybe there is something that you forgot to add or that should be added.
Below I post my full calculator module:
module Calculator where
import Text.Read (readEither)
data Expr = Lit Int
| Add Expr Expr
| Sub Expr Expr
| Mul Expr Expr
| Div Expr Expr
eval :: Expr -> Int
eval expr =
case expr of
Lit num -> num
Add arg1 arg2 -> eval' ( + ) arg1 arg2
Sub arg1 arg2 -> eval' ( - ) arg1 arg2
Mul arg1 arg2 -> eval' ( * ) arg1 arg2
Div arg1 arg2 -> eval' div arg1 arg2
where
eval' :: (Int -> Int -> Int) -> Expr -> Expr -> Int
eval' operator arg1 arg2 = operator (eval arg1) (eval arg2)
parse :: String -> Either String Expr
parse str =
case parse' (words str) of
Left err -> Left err
Right (e, []) -> Right e
Right (_, rest) -> Left $ "Found extra tokens: " <> (unwords rest)
parse' :: [String] -> Either String (Expr, [String])
parse' [] = Left "unexpected end of expression"
parse' (token:rest) =
case token of
"+" -> parseBinary Add rest
"*" -> parseBinary Mul rest
"-" -> parseBinary Sub rest
"/" -> parseBinary Div rest
lit ->
case readEither of
Left err -> Left err
Right lit' -> Right (Lit lit', rest)
parseBinary :: (Expr -> Expr -> Expr) -> [String] -> Either String (Expr, [String])
parseBinary exprConstructor args =
case parse' args of
Left err -> Left err
Right (firstArg, rest') ->
case parse' rest' of
Left err -> Left err
Right (secondArg, rest'') ->
Right $ (exprConstructor firstArg secondArg, rest'')
run :: String -> String
run expr =
case parse expr of
Left err -> "Error: " <> err
Right expr' ->
let answer = show $ eval expr'
in "The answer is: " <> answer