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

24 Dec 2007

      Brandon S. Allbery KF8NH wrote:
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On Dec 24, 2007, at 13:18 , Isaac Dupree wrote:
...
Paulo J. Matos wrote:
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On Dec 23, 2007 12:44 PM, Isaac Dupree  wrote:
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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???
when the first one returns Nothing, the whole expression becomes 
Nothing without examining the later parts of computation (as Chaddaï 
said)
One thng that's not obvious here is that pattern match failure 
translates to a call to "fail", which in the definition of Monad for 
Maybe becomes Nothing.
(Hm.  Isaac:  I thought that translation only happened for the "do" 
sugar, and in the direct case you must do it yourself or Haskell raises 
the "incomplete pattern match" exception?)
Tuple-matching never fails (except for _|_) -- there's only one 
constructor.  In this case it's only the intrinsic failure of 
BS.readInt.  You're thinking of something like

  do
    [a,b] <- readListOfInts foo
    return (a+b)
--readListOfInts is a function I made up :: String -> Maybe [Int]

which can fail
(1) if readListOfInts returns Nothing
(2) because of the do-notation, also if the list doesn't have exactly 
two elements in it.

Isaac