Ryan Bloor wrote:

I have a function parseInt... which needs an error guard for when the input is not an Int.

parseInt :: ParserparseInt [] = []parseInt xs = let (digits, rest) = span isDigit (removeSpace xs) in [(EInt (read digits), removeSpace rest)]

This is how your source code happens to be rendered on my mail client. This part of your mail is the reason:

--===============0328857386== Content-Type: multipart/alternative; boundary="_4752b91d-a167-4274-af0a-1982e53bc6c0_"

--_4752b91d-a167-4274-af0a-1982e53bc6c0_ Content-Type: text/plain; charset="Windows-1252" Content-Transfer-Encoding: quoted-printable

This encoding (charset="Windows-1252") is not available on my system. It would help others to read your message (leading to more responses to your question and thus better help) if you could configure your mail program to use a more widely understood encoding, for instance utf-8, or latin-1.

Also... I have a function that does this... parseBinaryOp "+" "(5 + 2) if" gives...[(Int 5, Int 2, "if")] so, op is '+' or "&&". I am unsure of how to begin...

parseBinaryOp :: String -> String -> [(Expr, Expr, String)]parseBinaryOp op str

Thankyou

Ryan

Thank you, too, for respecting mailing list netiquette :-)) Now to your question: Since Haskell is pure, you need to encode possible parse failure into the result type of your parsing function, like this: data Result = Success Expr | Failure String where the 'Expr' above is the type you want to get back on a successful parse, and the type argument 'String' to the 'Failure' constructor is for error messages. Your parseExpr function could then have a type like type Parser = String -> ParseResult Expr which could be called (from main) thus case parseExpr input of Success expr -> print expr Failure err_msg -> putStrLn $ "parse error: " ++ err_msg Another good idea, instead of just collecting a list (in fact, a tuple) of tokens as you did above, is to design 'Expr' so that it can represent a complete parse _tree_. Since usually expressions can be nested arbitrarily deep, any finite type won't do, you need a recursive type. For example data BinOp = OpPlus | OpMinus | OpMult -- enumerating all possible operators is more type safe than using String -- an expression is... Expr = EInt Integer -- either a literal integer | EBinOp BinOp Expr Expr -- or two expressions combined with a binary operator Hope this helps. Cheers Ben