On Mon, Oct 6, 2008 at 8:07 PM, Christian Maeder
Slavomir Kaslev wrote:
freeParser = freeParser' minBound where enumAll' :: (Bounded a, Enum a) => a -> [a] enumAll' _ = enumAll freeParser' :: (Enum a, Bounded a, Show a, Read a) => a -> Parser a freeParser' x = liftM read $ choice (map (string . show) (enumAll' x))
1. I would use an explicit function argument instead of "Show" to allow strings starting with lower case.
You are right. But that was not the problem. The problem was that I wrestled with Haskell's type system quite a bit to make freeParser work. What I want to write is freeParser :: (Enum a, Bounded a, Show a, Read a) => Parser a freeParser = liftM read $ choice (map (string . show) enumAll) but it doesn't compile. How can I make this piece code work?
2. Calling read after parsing looks stupid. Just return the value shown as parser result (within map).
Good point. It is silly =-)
3. Instead of the "string" parser, it should be checked if a further alphaNum or '_' follows (for the longest match). And don't forget "try"!
Sure. I actually use Parsec's reserved, which is kind enough to manage all this stuff for me.
Cheers Christian
[Actually, in my code I use reserved' (reserved' x = reserved x >> return x) instead of string, where reserved is from Parsec's builtin tokenizer (which does some neat things behind the curtains). Here string is used just to illustrate the expamle.]
The problem is that freeParser, although useful, is far from elegant. It's something that I came up with by trial and error. In short: it's a hack.
I would like to hear your suggestions about how it can be beautified.
Thank you in advance.
Cheers!
-- Slavomir Kaslev