Output with precision 5

app/Main.hs

@@ -2,16 +2,31 @@ module Main (main) where
 
 import Text.Parsec
 
-import Lib (exprparser, evaluate)
+import Lib (exprparser, evaluate, replaceVars)
+
+import Data.Map (Map)
+import qualified Data.Map as Map
+import Data.Ratio
 
 import System.IO
 
 main :: IO ()
 main = ioLoop
 
-useResult (Right e) = show . evaluate $ e
+precision = 5 :: Int
+
+showRatio :: Int -> Rational -> String
+showRatio p r = (if (r < 0) then "-" else "") ++ prepoint_digits ++ "." ++ postpoint_digits
+    where
+    	prepoint_digits  = init . show . round $ (r * 10)
+	postpoint_digits = (take p) . (drop (length prepoint_digits)) . show . round $ (r * 10^p)
+
+useResult (Right e) = (showRatio precision) . evaluate $ replaceVars e vars
 useResult (Left  e) = show e
 
+vars :: Map String Rational
+vars = Map.fromList [("pi", 245850922 % 78256779), ("e", 271801 % 99990)]
+
 ioLoop :: IO ()
 ioLoop = do done <- isEOF
             if done

hc.cabal

@@ -35,6 +35,7 @@ library
   ghc-options: -Wall -Wcompat -Widentities -Wincomplete-record-updates -Wincomplete-uni-patterns -Wmissing-export-lists -Wmissing-home-modules -Wpartial-fields -Wredundant-constraints
   build-depends:
       base >=4.7 && <5
+    , containers
     , parsec
   default-language: Haskell2010
 
@@ -49,6 +50,7 @@ executable hc-exe
   ghc-options: -Wall -Wcompat -Widentities -Wincomplete-record-updates -Wincomplete-uni-patterns -Wmissing-export-lists -Wmissing-home-modules -Wpartial-fields -Wredundant-constraints -threaded -rtsopts -with-rtsopts=-N
   build-depends:
       base >=4.7 && <5
+    , containers
     , hc
     , parsec
   default-language: Haskell2010
@@ -65,6 +67,7 @@ test-suite hc-test
   ghc-options: -Wall -Wcompat -Widentities -Wincomplete-record-updates -Wincomplete-uni-patterns -Wmissing-export-lists -Wmissing-home-modules -Wpartial-fields -Wredundant-constraints -threaded -rtsopts -with-rtsopts=-N
   build-depends:
       base >=4.7 && <5
+    , containers
     , hc
     , parsec
   default-language: Haskell2010

package.yaml

@@ -22,6 +22,7 @@ description:         Please see the README on GitHub at <https://github.com/gith
 dependencies:
 - base >= 4.7 && < 5
 - parsec
+- containers
 
 ghc-options:
 - -Wall

src/Lib.hs

@@ -1,9 +1,11 @@
 module Lib
-    ( exprparser, evaluate
+    ( exprparser, evaluate, replaceVars 
     ) where
 import Control.Applicative((<*))
 
 import Data.Ratio
+import Data.Map (Map)
+import qualified Data.Map as Map
 
 import Text.Parsec
 import Text.Parsec.Char
@@ -26,7 +28,7 @@ def = emptyDef{ commentStart = ""
               , opStart = oneOf "+-/*^"
               , opLetter = oneOf "+-/*^"
               , reservedOpNames = ["+", "-", "/", "*", "^"]
-              , reservedNames = ["pi", "e"]
+              , reservedNames = []
               }
 
 TokenParser{ parens = m_parens
@@ -64,13 +66,14 @@ constantInteger = try (do
 
 constantRational :: Parser Rational
 constantRational = do
-    natural <- m_natural
-    _       <- char '.'
-    decimal <- m_natural
+    natural        <- m_natural
+    _              <- char '.'
+    decimal_digits <- many digit
+    let decimal = read decimal_digits :: Integer
     let natural_length = length . show $ natural
-    let decimal_length = length . show $ decimal
+    let decimal_length = length decimal_digits
     let numerator   = natural * (10 ^ decimal_length) + decimal
-    let denominator = 10 ^ (decimal_length + natural_length - 2)
+    let denominator = 10 ^ (decimal_length + natural_length - 1)
     return (numerator % denominator)
 
 {-
@@ -88,6 +91,11 @@ term = m_parens exprparser
        <|> fmap Constant constantInteger
        <|> fmap Constant constantRational
 
+replaceVars :: Expr -> Map.Map String Rational -> Expr
+replaceVars (Variable name) vs = Constant . maybe (0 % 1) id $ Map.lookup name vs
+replaceVars (Binary op a b) vs = Binary op (replaceVars a vs) (replaceVars b vs) 
+replaceVars (Constant c)    vs = Constant c
+
 evaluate :: Expr -> Rational
 evaluate (Constant c) = c
 evaluate (Binary Plus     a b) = evaluate a + evaluate b