[Haskell-cafe] advice on a parsing function

minh thu noteed at gmail.com
Wed Mar 11 13:38:04 EDT 2009

2009/3/11 minh thu <noteed at gmail.com>:
> 2009/3/11 Manlio Perillo <manlio_perillo at libero.it>:
>> minh thu ha scritto:
>>> [...]
>>> I suggest you try an alternative strategy.
>>> That altenrative strategy is twofold, just like you have
>>> quiz' and quiz'.
>>> This alternate strategy avoid pattern matching on strings
>>> (which would be cumbersome for a bit more complex syntax).
>> But for this specific case it is very compact and elegant (IMHO).
> I would say it is difficult to see what you're doing in the code without
> the desciption you gave in the mail. But you're right, it's not the string
> pattern matching which is the problem.
> It is more the pair (Int, rest of the bytestring which can begin or
> not with ':')...
> Why not have quiz' accepting just the bytestring (and not the id value),
> and returning the (Int,[Int]) ?
>>> [...]
>>> Now, given those two functions, try to apply them
>>> on your input string, feeding the next function application
>>> with the resulting string of the current application.
>> So, I should not split the string into lines?
> See below.
>> An useful feature of my program is that it parses both an input like:
>> 1:
>> 1046323,2005-12-19
>> and
>> 1:
>> 1046323
>> If I write a parser from scratch I need to implement two separate functions.
> I didn't think to that but nothing prevent you to write the second function I
> suggested to account for that case, or for an end of line (if can 'eat' the ':'
> from the input, you can also eat a newline).
> Thu


The approach I suggested is a bit overkill. You can indeed use L.lines
to split the input into lines then work on that.

But still, avoid the pair (Int, Bytestring). Instead, you can basically map
on each line the unsafeReadInt modified to :
- return the id
- return if it is one kind of id or the other kind.

so :
type UserId = Int
type MovieId = Int
unsafeReadInt :: Line -> Either MovieId UserId

Now you have a nice list [Either MovieId UserId] that
you need to transform into (MovieId, [UserId]).

Sorry, for the previous response.


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