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

[Conal Elliott]

To clean up even more, use StateT B.ByteString Maybe.  Then the ByteString
threading will be invisible, leading to just "liftM2 (,) readI readI", for
suitably defined readI.

On Dec 23, 2007 6:45 AM, Bryan O'Sullivan <bos at serpentine.com> wrote:

> Paulo J. Matos wrote:
>
> > I guess the latter is the correct guess.
>
> Good guess!
>
> You can take advantage of the fact that the Maybe type is an instance of
> the Monad typeclass to chain those computations together, getting rid of
> all of the explicit case analysis.
>
> import qualified Data.ByteString.Char8 as B
> import Data.Char (isDigit)
>
> readTwoInts :: B.ByteString -> Maybe ((Int, Int), B.ByteString)
> readTwoInts r = do
>  (a, s) <- B.readInt . B.dropWhile (not . isDigit) $ r
>  (b, t) <- B.readInt . B.dropWhile (not . isDigit) $ s
>  return ((a, b), t)
>
> Let's try that in ghci:
>
>  *Main> readTwoInts (B.pack "hello 256 299 remainder")
>  Just ((256,299)," remainder")
>
> The case analysis is still happening, it's just being done behind your
> back by the (>>=) combinator, leaving your code much tidier.  (And why
> is there no explicit use of (>>=) above?  Read about desugaring of "do"
> notation in the Haskell 98 report.)
>
> The learning you'll want to do, to be able to reproduce code such as the
> above, is about monads.
>
> Cheers,
>
>        <b
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