September 2017 Update

Task/Arithmetic-evaluation/Haskell/arithmetic-evaluation.hs

@@ -1,28 +1,47 @@
+{-# LANGUAGE FlexibleContexts #-}
+
 import Text.Parsec
 import Text.Parsec.Expr
 import Text.Parsec.Combinator
 import Data.Functor
 import Data.Function (on)
 
-data Exp = Num Int
-         | Add Exp Exp
-         | Sub Exp Exp
-         | Mul Exp Exp
-         | Div Exp Exp
+data Exp
+  = Num Int
+  | Add Exp
+        Exp
+  | Sub Exp
+        Exp
+  | Mul Exp
+        Exp
+  | Div Exp
+        Exp
 
+expr
+  :: Stream s m Char
+  => ParsecT s u m Exp
 expr = buildExpressionParser table factor
-    where table = [[op "*" (Mul) AssocLeft, op "/" (Div) AssocLeft]
-                  ,[op "+" (Add) AssocLeft, op "-" (Sub) AssocLeft]]
-          op s f assoc = Infix (f <$ string s) assoc
-          factor =  (between `on` char) '(' ')' expr
-                <|> (Num . read <$> many1 digit)
+  where
+    table =
+      [ [op "*" Mul AssocLeft, op "/" Div AssocLeft]
+      , [op "+" Add AssocLeft, op "-" Sub AssocLeft]
+      ]
+    op s f = Infix (f <$ string s)
+    factor = (between `on` char) '(' ')' expr <|> (Num . read <$> many1 digit)
 
-eval :: Num a => Exp -> a
-eval (Num x)   = fromIntegral x
-eval (Add a b) = eval a   +   eval b
-eval (Sub a b) = eval a   -   eval b
-eval (Mul a b) = eval a   *   eval b
+eval
+  :: Integral a
+  => Exp -> a
+eval (Num x) = fromIntegral x
+eval (Add a b) = eval a + eval b
+eval (Sub a b) = eval a - eval b
+eval (Mul a b) = eval a * eval b
 eval (Div a b) = eval a `div` eval b
 
-solution :: Num a => String -> a
+solution
+  :: Integral a
+  => String -> a
 solution = either (const (error "Did not parse")) eval . parse expr ""
+
+main :: IO ()
+main = print $ solution "(1+3)*7"