replacing Variables with constant values

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
@@ -89,6 +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