stack setup and integer expression parser

src/Lib.hs

@@ -0,0 +1,58 @@
+module Lib
+    ( 
+    ) where
+import Control.Applicative((<*))
+
+import Data.Ratio
+
+import Text.Parsec
+import Text.Parsec.Char
+import Text.Parsec.String
+import Text.Parsec.Expr
+import Text.Parsec.Token
+import Text.Parsec.Language
+
+data Expr = Var String | Constant Integer | Unary UnaryOperator Expr | Binary BinaryOperator Expr Expr
+    deriving Show
+data UnaryOperator = Not
+    deriving Show
+data BinaryOperator = Plus | Minus | Multiply | Divide
+    deriving Show
+
+naturalRatio a = a % 1
+
+def = emptyDef{ commentStart = ""
+              , commentEnd = ""
+              , identStart = letter <|> char '_'
+              , identLetter = alphaNum <|> char '_'
+              , opStart = oneOf "+-/*"
+              , opLetter = oneOf "+-/*"
+              , reservedOpNames = ["+", "-", "/", "*"]
+              , reservedNames = ["pi", "e"]
+              }
+
+TokenParser{ parens = m_parens
+           , identifier = m_identifier
+           , reservedOp = m_reservedOp
+           , reserved   = m_reserved
+           , semiSep1   = m_semiSep1
+           , natural    = m_natural
+           , whiteSpace = m_whiteSpace } = makeTokenParser def
+
+exprparser :: Parser Expr
+exprparser = buildExpressionParser table term <?> "expression"
+
+table = [
+            [
+                Infix (m_reservedOp "*" >> return (Binary Multiply)) AssocLeft,
+                Infix (m_reservedOp "/" >> return (Binary Divide)) AssocLeft
+            ],
+            [
+                Infix (m_reservedOp "+" >> return (Binary Plus)) AssocLeft,
+                Infix (m_reservedOp "-" >> return (Binary Minus)) AssocLeft
+            ]
+        ]
+
+term = m_parens exprparser
+       <|> fmap Var m_identifier
+       <|> fmap Constant m_natural