[Haskell-cafe] Re: How to give unique name/id to nodes outside any monad ?

[minh thu]

2009/1/8 Ertugrul Soeylemez <es at ertes.de>:
> "minh thu" <noteed at gmail.com> wrote:
>
>> Nothing, simply the notation. Now, with the remark of Luke, I'm
>> wondering how bad it is to use makeStableName/hashStableName to "copy"
>> the data structure in a similar one with explicit reference (that is,
>> using pointer or keys in a map or whatever).
>
> Probably you're misusing the notation.  I don't see any reason, why
> monadic notation should be less readable.  Usually it's even more
> readable.  Luke's remark is very valid.  Haskell is the wrong language
> for imperative programming.  You don't have _any_ benefit of Haskell, if
> you use it like C.  Try to change your mind.  Monads aren't evil.  They
> are there to make your life easier.  Way easier than imperative methods.

Well, maybe it's is just my opinion, but I found the non-monadic code
in the previous mail
easier to write than the monadic one... I don't know against what
you're making the compareason to say it's more readable.

Although I agree using Haskell requires some change of thinking,
statement like yours
are a bit too much for me. I find Haskell a nice language even for
imperative programming...

Cheers,
Thu

> Greets,
> Ertugrul.
>
>
>> Thank you,
>> Thu
>>
>> 2009/1/8 Lennart Augustsson <lennart at augustsson.net>:
>> > Look at http://www.haskell.org/ghc/docs/latest/html/libraries/base/System-Mem-StableName.html.
>> >
>> > But what's wrong with constructing the graph in a monad?
>> >
>> > On Thu, Jan 8, 2009 at 9:53 AM, minh thu <noteed at gmail.com> wrote:
>> >> Well, the processing of the data structure has to be done in the IO monad.
>> >> What is the library you talk about ? Could it give the "stable names"
>> >> (in IO) for
>> >> each node of the mentioned graph (I mean, after the graph has been constructed
>> >> purely) ?
>> >>
>> >> Thanks,
>> >> Thu
>> >>
>> >> 2009/1/8 Lennart Augustsson <lennart at augustsson.net>:
>> >>> Of course you don't need a monad, but you need to do the same
>> >>> operations as you would with a state monad to number the nodes.  This
>> >>> is the only way in (pure) Haskell.  There is no object identity in
>> >>> Haskell, so if you want the nodes to have identity you need to provide
>> >>> it.
>> >>>
>> >>> GHC does have a library for stable names which (in the IO monad)
>> >>> allows you to get something akin to the address of a value in memory.
>> >>> But that's not the functional way of doing this.
>> >>>
>> >>>  -- Lennart
>> >>>
>> >>> On Thu, Jan 8, 2009 at 9:28 AM, minh thu <noteed at gmail.com> wrote:
>> >>>> Hi,
>> >>>>
>> >>>> I'd like to process some kind of graph data structure,
>> >>>> say something like
>> >>>>
>> >>>> data DS = A [DS] | B DS DS | C.
>> >>>>
>> >>>> but I want to be able to discover any sharing.
>> >>>> Thus, in
>> >>>>
>> >>>> b = B a a where a = A [C],
>> >>>>
>> >>>> if I want to malloc a similar data structure,
>> >>>> I have to handle to the node representing B
>> >>>> two times the same pointer (the one returned
>> >>>> after allocating A [C]).
>> >>>>
>> >>>> To discover sharing, I thought it would be
>> >>>> necessary to give unique name to node and
>> >>>> then compare them while traversing the graph.
>> >>>> I could give the name by hand but it would be
>> >>>> cumbersome. But at least it would not require
>> >>>> any monad for the bookkeeping of ungiven
>> >>>> names. Is it possible to give those names
>> >>>> automatically but outside any monad ?
>> >>>>
>> >>>> Thanks,
>> >>>> Thu
>> >>>> _______________________________________________
>> >>>> Haskell-Cafe mailing list
>> >>>> Haskell-Cafe at haskell.org
>> >>>> http://www.haskell.org/mailman/listinfo/haskell-cafe
>> >>>>
>> >>>
>> >>
>> >
>
>
>
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