variables

src/Lib.hs

@@ -1,5 +1,5 @@
 module Lib
-    ( exprparser, evaluate, replaceVars, Expr
+    ( exprparser, evaluate, replaceVars, Expr, extractVariableDefinitions
     ) where
 import Control.Applicative((<*))
 
@@ -14,7 +14,7 @@ import Text.Parsec.Expr
 import Text.Parsec.Token
 import Text.Parsec.Language
 
-data Expr = Variable String | Constant Rational | Binary BinaryOperator Expr Expr
+data Expr = Variable String | Constant Rational | Binary BinaryOperator Expr Expr | VariableDefinition String Rational
     deriving Show
 data BinaryOperator = Plus | Minus | Multiply | Divide | Power
     deriving Show
@@ -73,9 +73,42 @@ constantRational = do
     let natural_length = length . show $ natural
     let decimal_length = length decimal_digits
     let numerator   = natural * (10 ^ decimal_length) + decimal
-    let denominator = 10 ^ (decimal_length + natural_length - 1)
+    let denominator = 10 ^ decimal_length
     return (numerator % denominator)
 
+variableDefinition :: Parser Expr
+variableDefinition = try (do
+        name  <- m_identifier
+        _     <- char '='
+        value <- constantInteger <|> constantRational
+        return (VariableDefinition name value)
+    )
+
+term = m_parens exprparser
+       <|> variableDefinition
+       <|> fmap Variable m_identifier
+       <|> fmap Constant constantInteger
+       <|> fmap Constant constantRational
+
+extractVariableDefinitions :: Expr -> Map.Map String Rational
+extractVariableDefinitions (VariableDefinition name value) = Map.singleton name value
+extractVariableDefinitions (Binary _ a b)                  = Map.union (extractVariableDefinitions a) (extractVariableDefinitions b)
+extractVariableDefinitions (Constant _)                    = Map.empty
+extractVariableDefinitions (Variable _)                    = Map.empty
+
+replaceVars :: Expr -> Map.Map String Rational -> Either String Expr
+replaceVars (Constant c)                  vs = Right $ Constant c
+replaceVars (Variable name)               vs = maybe (Left ("Usage of unknown variable: '" ++ name ++ "'\nSuggestion: define the variable\n++  " ++ name ++ "=42")) (Right . Constant . id) $ Map.lookup name vs
+replaceVars (VariableDefinition _ value ) vs = Right $ Constant value
+replaceVars (Binary op l r)               vs = case leftBranch of
+        Left  s -> Left  s
+        Right a -> case rightBranch of
+            Left s  -> Left s
+            Right b -> Right $ Binary op a b
+    where
+        leftBranch  = replaceVars l vs
+	rightBranch = replaceVars r vs
+
 {-
  - a/b ^ c/d
  - (a ^ c/d) / b ^ (c/d)
@@ -87,23 +120,6 @@ rationalPower a b = rationalPower' (numerator a, denominator a) (numerator b, de
         rationalPower' (a, b) (c, 1) = a ^ c % b ^ c
         rationalPower' _ _ = error "Powers with unnatural numbers are not supported yet"
 
-term = m_parens exprparser
-       <|> fmap Variable m_identifier
-       <|> fmap Constant constantInteger
-       <|> fmap Constant constantRational
-
-replaceVars :: Expr -> Map.Map String Rational -> Either String Expr
-replaceVars (Constant c)    vs = Right $ Constant c
-replaceVars (Variable name) vs = maybe (Left ("Usage of unknown variable: '" ++ name ++ "'\nSuggestion: define the variable\n++  " ++ name ++ "=42")) (Right . Constant . id) $ Map.lookup name vs
-replaceVars (Binary op l r) vs = case leftBranch of
-        Left  s -> Left  s
-        Right a -> case rightBranch of
-            Left s  -> Left s
-            Right b -> Right $ Binary op a b
-    where
-        leftBranch  = replaceVars l vs
-	rightBranch = replaceVars r vs
-
 evaluate :: Expr -> Rational
 evaluate (Constant c) = c
 evaluate (Binary Plus     a b) = evaluate a + evaluate b