add basic literate lamb (lilamb) parsing

lamb.hs

@@ -4,10 +4,11 @@
 
 import System.Environment (getArgs)
 import System.Directory (doesFileExist)
+import System.FilePath (FilePath, splitExtension)
 import Interp (evalProgram, evalString, Value(UnitV))
 
 -- returns Nothing if all files exist, or Just path for the first one that doesn't
-allExist :: [String] -> IO (Maybe String)
+allExist :: [FilePath] -> IO (Maybe FilePath)
 allExist [] = return Nothing
 allExist ("-":xs) = allExist xs
 allExist (x:xs) = do
@@ -15,10 +16,20 @@ allExist (x:xs) = do
 	if exists then allExist xs
 	else return $ Just x
 
+isLiterate :: FilePath -> Bool
+isLiterate path = snd (splitExtension path) == ".lilamb"
+
+-- Takes the lines of a literate program and returns the lines for a new executable program
+-- from lines beginning with four spaces.
+parseLiterate :: [String] -> [String]
+parseLiterate lns = [drop 4 line | line <- lns, take 4 line == "    "]
+
 evalFile :: String -> IO Value
 evalFile path = do
 	contents <- if path == "-" then getContents else readFile path
-	evalString contents
+	if isLiterate path then
+		evalString . unlines . parseLiterate . lines $ contents
+	else evalString contents
 
 main = do
 	args <- getArgs