Parse for loops

Watch out for the <++: this tries to parse whatever is on the left first,
and then only if it fails tries whatever is on the right. This is
different from <|>, which will include both in all the possible
final parsings.

diff --git a/AST.hs b/AST.hs
index ae18a52347ce91c37336a06f891bae690ba691ec..b019a2a2078f91c18a5046f63e10676f2c40d5aa 100644 (file)
--- a/AST.hs
+++ b/AST.hs
@@ -2,13 +2,14 @@ module AST where
 
 import Data.Char
 import Text.Read 
-import Text.ParserCombinators.ReadP hiding ((+++), choice)
+import Text.ParserCombinators.ReadP hiding ((+++), (<++), choice)
 
 data Expr = Num Double
           | Var String
           | BinOp BinOp Expr Expr
           | Call String [Expr]
           | If Expr Expr Expr
+          | For String Expr Expr (Maybe Expr) Expr
   deriving Show
 
 data BinOp = Add | Sub | Mul | Cmp Ordering
@@ -19,6 +20,7 @@ instance Read Expr where
                              , parseVar
                              , parseCall
                              , parseIf
+                             , parseFor
                              , parseBinOp "<" 10 (Cmp LT)
                              , parseBinOp ">" 10 (Cmp GT)
                              , parseBinOp "==" 10 (Cmp EQ)
@@ -47,6 +49,18 @@ instance Read Expr where
             spaced $ string "else"
             elseE <- readPrec
             return (If cond thenE elseE)
+          parseFor = do
+            spaced $ string "for"
+            identifier <- lift (munch1 isAlpha)
+            spaced $ char '='
+            start <- readPrec
+            spaced $ char ','
+            cond <- readPrec
+            stp <- (spaced (char ',') >> Just <$> step readPrec)
+                     <++ pure Nothing
+            spaced $ string "in"
+            body <- readPrec
+            return (For identifier start cond stp body)
           spaced f = lift $ skipSpaces >> f >> skipSpaces
             
 data Prototype = Prototype String [String]