[Haskell-beginners] question, chapter 10 Real World Haskell

Michael Mossey mpm at alumni.caltech.edu
Sat Apr 11 10:41:50 EDT 2009

Quentin Moser wrote:
> On Sat, 11 Apr 2009 02:02:04 -0700
> Michael Mossey <mpm at alumni.caltech.edu> wrote:
> This is a matter of good coding style (in the sense of modularity,
> maintanability, etc.).
> As you've seen it, the authors have first
> started with a (>>?) function that allows them to combine Maybes, that
> is to combine functions with a possibility of error. 

Thanks for the detailed explanation. I like how you show that the original >>? 
function, and the method of signaling either an error or a result, could have been 
written with ==> notation.

But, I'm still confused on a point. Let me put it this way. The authors wrote:

-- file: ch10/Parse.hs
-- import the Word8 type from Data.Word
parseByte :: Parse Word8
parseByte =
     getState ==> \initState ->
     case L.uncons (string initState) of
       Nothing ->
           bail "no more input"
       Just (byte,remainder) ->
           putState newState ==> \_ ->
           identity byte
         where newState = initState { string = remainder,
                                      offset = newOffset }
               newOffset = offset initState + 1

Why couldn't they have avoided getState by writing

parseByte = Parse (
   \initState ->
   case L.unconcs (string initState) of
               newOffset = offset initState +1

...because a parser is by definition a function that takes ParseState as its input. I 
understand you are saying that in future chapters they may introduce new capabilities 
that fit within this framework, so maybe that's what I'm not seeing. Maybe they will 
redefine getState (or something equivalent) so that it does more than grab the 
unchanged state from the Parse.

But I'm curious to know if my second implementation works the same as the first, in 


More information about the Beginners mailing list