Re: [Haskell-cafe] Re: Comments on reading two ints off Bytestring

2007/12/24, Paulo J. Matos :

On Dec 24, 2007 11:55 AM, Paulo J. Matos wrote:

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On Dec 23, 2007 12:44 PM, Isaac Dupree wrote:

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-- this should work too parseHeader3 :: BS.ByteString -> Maybe (Int, Int) --note accurate type signature, which helps us use Maybe failure-monad, --although losing your separate error messages

Oh gee, I just noticed that my type sig is in fact not correct. How come GHC doesn't complain?

Your type is correct.

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parseHeader3 bs = do (x, rest) <- BS.readInt $ BS.dropWhile (not . isDigit) bs (y, _) <- BS.readInt $ BS.dropWhile (not . isDigit) rest return (x, y)

What happens then if the first BS.readInt return Nothing???

Ok, got it, I'm not returning a maybe. That's it then. Still, the first question remains... what happens to (x, rest) if BS.readInt returns Nothing.

Your function return a Maybe (Int,Int), (x,y) is of type (Int,Int). If readInt return a nothing, the whole funtion will return a Nothing per (>>=) instance definition. -- Jedaï