add task 3

exercise2/part 2/Main.hs

+module Main where
+
+import Tokens
+import System.Environment
+import Control.Exception
+import System.IO
+
+main :: IO ()
+main = catch main' noLex
+
+main' = do (fileName : _) <- getArgs
+           sourceText <- readFile fileName
+           putStrLn $ "Lexing: " ++ sourceText
+           let lexedProg = alexScanTokens sourceText
+           putStrLn $ "Lexed as: " ++ (show lexedProg)
+
+noLex :: ErrorCall -> IO ()
+noLex e = do let err = show e
+             hPutStrLn stderr ("Problem with lexing :" ++ err)
+             return ()
\ No newline at end of file

exercise2/part 2/Tokens.x

+{ 
+module Tokens where 
+}
+
+%wrapper "posn" 
+$digit = 0-9     
+-- digits 
+$alpha = [a-zA-Z]    
+-- alphabetic characters
+
+tokens :-
+  $white+     ; 
+  "--".*      ; 
+  let         { \p s -> TokenLet p} 
+  in          { \p s -> TokenIn p}
+  $digit+     { \p s -> TokenInt p (read s) } 
+  \=          { \p s -> TokenEq p}
+  \+          { \p s -> TokenPlus p}
+  \-          { \p s -> TokenMinus p}
+  \*          { \p s -> TokenTimes p}
+  \/          { \p s -> TokenDiv p}
+  \(          { \p s -> TokenLParen p}
+  \)          { \p s -> TokenRParen p}
+  \^          { \p s -> TokenExp p}
+  $alpha [$alpha $digit \_ \’]*   { \p s -> TokenVar p s } 
+
+{ 
+-- Each action has type :: AlexPosn -> String -> Token 
+-- The token type: 
+data Token = 
+  TokenLet AlexPosn         | 
+  TokenIn AlexPosn          | 
+  TokenInt AlexPosn Int     |
+  TokenVar AlexPosn String  | 
+  TokenEq AlexPosn          |
+  TokenPlus AlexPosn        |
+  TokenMinus AlexPosn       |
+  TokenTimes AlexPosn       |
+  TokenDiv AlexPosn         |
+  TokenExp AlexPosn         |
+  TokenLParen AlexPosn      |
+  TokenRParen AlexPosn       
+  deriving (Eq,Show) 
+
+tokenPosn :: Token -> String
+tokenPosn (TokenLet (AlexPn _ l c)) = (show l) ++ " : " ++ (show c)
+-- all other cases as well
+
+}
\ No newline at end of file

exercise2/part 2/test.txt

+let x = 34 in x + 17
\ No newline at end of file

exercise2/part 3/MDLTokens.x

+{ 
+module MDLTokens where 
+}
+
+-- start with writing a grammar
+
+-- E := Forward n | Rotate D | Check [1..9] | if E then E else E | E ; E | ( E )
+-- D := L | R
+
+%wrapper "posn"
+$digit = 0-9
+
+tokens :-
+	$white+    ;
+	"--".*     ;
+	Forward    {\p s -> TokenForward p}  
+	Rotate     {\p s -> TokenRotate p}
+	Check      {\p s -> TokenCheck p}
+	if         {\p s -> TokenIf p}
+	then       {\p s -> TokenThen p}
+	else       {\p s -> TokenElse p}
+	L          {\p s -> TokenLeft p}
+	R          {\p s -> TokenRight p}
+	\;
+	\(
+	\)
+	[1..9]     {\p s -> TokenDigit p (read s)}
+	$digit $digit+ {\p s -> TokenInt p (read s)}
+
+{
+	data MDLToken = 
+}
\ No newline at end of file