[Haskell-cafe] wondering about a MonadIO instance for a heap data type

[Eugene Kirpichov]

List is a monad because it has an associated way of sequencing
computations: namely, "collect results of the second computation
invoked on all results of the first computation". That's not because
List is a data structure (similarly to Heap), it's because it is
associated with the computational abstraction of computations with
multiple results. The heap data structure does not have an associated
computational abstraction, at least not that one I'm aware of, and
definitely not one that would make any sense in the context of
heapsort. So it doesn't make sense to pretend that it does.

On Wed, Jul 11, 2012 at 5:00 PM, Qi Qi <qiqi789 at gmail.com> wrote:
> List [] is a monad, why not for heap data. Heap data could be an instance of
> Monad too.
> I have the heapsort function, and just wanted to rewrite a verbose version
> of it by using liftIO.
> But I would look into Debug.Trace. Thanks for your hint.
>
> Qi
>
>
> On Wednesday, July 11, 2012 5:28:17 PM UTC-5, Eugene Kirpichov wrote:
>>
>> Use Debug.Trace.
>> It does not make sense to declare that heap is a monad, as a monad is an
>> abstraction of sequencing computations, and a heap is not an abstraction of
>> sequencing computations at all. You don't make your String class implement
>> the "rendering engine" interface just because you want to use it in a
>> computer game program, equally you dont pretend that a heap is a way of
>> sequencing computations just because you want to sequence computations
>> related to heaps.
>>
>> The actual computation in your case is the heapsort function, not the
>> heap. If you absolutely must use IO, add IO to the functions type.
>>
>> 11.07.2012, в 15:19, Qi Qi <qiqi789 at gmail.com> написал(а):
>>
>> > Hi,
>> >
>> > I was wondering about creating an instance of MonadIO for a heap data.
>> > Any hints?
>> >
>> > data Heap a = E | T Int a (Heap a) (Heap a)
>> >     deriving (Eq, Ord, Read, Show)
>> >
>> > The reason is that I want to use liftIO during a heapsort to print out
>> > intermediate results.
>> >
>> > Thanks.
>> >
>> > Qi Qi
>> >
>> >
>> >
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-- 
Eugene Kirpichov
http://www.linkedin.com/in/eugenekirpichov