Support for nested variable definitions

src/Lib.hs

@@ -1,5 +1,5 @@
 module Lib
-    ( exprparser, evaluate, replaceVars, Expr, extractVariableDefinitions
+    ( exprparser, evaluate, replaceVars, Expr, extractVariableDefinitions, updateVariables
     ) 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 | VariableDefinition String Rational
+data Expr = Variable String | Constant Rational | Binary BinaryOperator Expr Expr | VariableDefinition String Expr
     deriving Show
 data BinaryOperator = Plus | Minus | Multiply | Divide | Power
     deriving Show
@@ -80,7 +80,7 @@ variableDefinition :: Parser Expr
 variableDefinition = try (do
         name  <- m_identifier
         _     <- char '='
-        value <- constantInteger <|> constantRational
+        value <- exprparser
         return (VariableDefinition name value)
     )
 
@@ -90,16 +90,29 @@ term = m_parens exprparser
        <|> 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
+extractVariableDefinitions :: Expr -> [(String,Expr)]
+extractVariableDefinitions (VariableDefinition name value) = [(name, value)]
+extractVariableDefinitions (Binary _ a b)                  = extractVariableDefinitions a  ++ extractVariableDefinitions b
+extractVariableDefinitions (Constant _)                    = []
+extractVariableDefinitions (Variable _)                    = []
+
+updateVariables :: [(String, Expr)] -> Map String Rational -> Either String (Map String Rational)
+updateVariables ds vs = foldl updateVariable (Right vs) ds
+
+updateVariable :: Either String (Map String Rational) -> (String, Expr) -> Either String (Map String Rational)
+updateVariable (Left  e)  _         = Left e
+updateVariable (Right vs) (name, e) = case updateVariables nvs vs of
+        Left e    -> Left e
+        Right uvs -> case replaceVars e uvs of
+            Left e    -> Left e
+            Right exp -> Right $ Map.insert name (evaluate exp) uvs
+    where
+        nvs = extractVariableDefinitions e
 
 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 (VariableDefinition name _)   vs = replaceVars (Variable name) vs
 replaceVars (Binary op l r)               vs = case leftBranch of
         Left  s -> Left  s
         Right a -> case rightBranch of
@@ -107,7 +120,7 @@ replaceVars (Binary op l r)               vs = case leftBranch of
             Right b -> Right $ Binary op a b
     where
         leftBranch  = replaceVars l vs
-	rightBranch = replaceVars r vs
+        rightBranch = replaceVars r vs
 
 {-
  - a/b ^ c/d