Question

At least I think that’s what’s going on.

Main.hs:

module Main (
    main
) where

import Arithmetic
import Data.Maybe
import Data.Either
import Control.Monad.Error

testExpr :: Expr Float
testExpr = 
        (MultExpr "*"
            (AddExpr "XXX"
                (NumExpr 1) 
                (AddExpr "-"
                    (NumExpr 24)
                    (NumExpr 21)
                )
            ) 
            (NumExpr 5) 
        )

main :: IO ()
main = do 
    putStrLn $ case eval testExpr of
            Left msg -> "Error: " ++ msg
            Right result -> show result

Arithmetic.hs:

{-# LANGUAGE GADTs #-}

module Arithmetic  where

type Op = String

data Expr a where
    NumExpr :: Float -> Expr Float
    AddExpr :: Op -> Expr Float -> Expr Float -> Expr Float
    MultExpr :: Op -> Expr Float -> Expr Float -> Expr Float

eval :: (Monad m) => Expr Float -> m Float
eval (NumExpr n) = return n
eval (AddExpr "+" e1 e2) = evalBin (+) e1 e2
eval (AddExpr "-" e1 e2) = evalBin (-) e1 e2
eval (AddExpr "%" e1 e2) = evalBin (%) e1 e2
eval (AddExpr _ _ _ ) = fail "Invalid operator. Expected +, - or %"
eval (MultExpr "*" e1 e2) = evalBin (*) e1 e2
eval (MultExpr "/" e1 e2) = evalBin (/) e1 e2
eval (MultExpr _ _ _ ) = fail "Invalid operator. Expected * or /"

evalBin :: (Monad m) => (Float -> Float -> Float) -> Expr Float -> Expr Float -> m Float
evalBin op e1 e2 = do 
  v1 <- eval e1
  v2 <- eval e2
  return $ op v1 v2

infixl 6 %
(%) :: Float -> Float -> Float
a % b = a - b * (fromIntegral $ floor (a / b))

But, when eval fails, I get an error in IO, without the “Error: ” string appended.

Answer 1

Which version of base are you using? fail is no longer defined to return a Left in the latest version of the Either e monad, instead it uses the default definition (which calls error, which throws an exception that can only be caught in IO).

I don’t know why this changed.