I am writing a Parsec parser for a C-like language and I have several datas that look more or less like this one:
import Control.Monad( liftM ) import Text.ParserCombinators.Parsec
data FooBar = Foo | Bar deriving (Show,Read,Bounded,Enum)
Looking at these, it seems that there should be no need to write a parser for this guy by hand. We already know that it is bounded and enumerable, so we can get a list of all possible FooBars:
enumAll :: (Bounded a, Enum a) => [a] enumAll = enumFromTo minBound maxBound
Also, we know how to show and read each FooBar. Therefore, I should get a free parser!
freeParser :: (Enum a, Bounded a, Show a, Read a) => Parser a
Here is one use of freeParser:
paramMod = option Foo freeParser test = parseTest $ do { x <- paramMod; eof; return x }
Not suprisingly: test "Foo" => Foo test "Bar" => Bar test "" => Foo I had a little hard time figuring out how this parser should look. The best I came up with was:
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))
[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