Paulo J. Matos wrote:
On Dec 23, 2007 12:44 PM, Isaac Dupree <[EMAIL PROTECTED]> wrote:
-- 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?
well, it is correct for Haskell if you want program failure for parse
failure... it's just not a _total_ function unless you use Maybe (which
determines whether you can have the code that uses parseHeader decide
what to do in the case of a failure)
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???
--or to be clearer without syntactic sugar, that is
parseHeader3 bs =
(BS.readInt $ BS.dropWhile (not . isDigit) bs)
>>= \(x, rest) ->
(BS.readInt $ BS.dropWhile (not . isDigit) rest)
>>= \(y, _) ->
return (x, y)
when the first one returns Nothing, the whole expression becomes Nothing
without examining the later parts of computation (as Chaddaï said)
Isaac
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