[Haskell-cafe] [Alternative] change some/many semantics

[wren ng thornton]

On 12/19/11 10:20 PM, David Menendez wrote:
> On Mon, Dec 19, 2011 at 6:37 PM, wren ng thornton<wren at freegeek.org>  wrote:
>> On 12/14/11 10:58 PM, Gregory Crosswhite wrote:
>>>
>>> Of course, this is not a simple change at all because it would have to
>>> be done in such a way as to respect the ordering of actions --- that
>>> is, we can't have each action executed only when the corresponding
>>> element of the list demanded is forced, or else actions would
>>> undesirably interleave.
>>
>> Therein lies the issue. To put this in a monadic context, this is the same
>> reason why we can't just say:
>>
>>     evalState (repeatM getNext) init
>>
>> e.g., to generate an infinite list of pseudorandom numbers and then discard
>> the final seed because we have all the numbers we'll ever need.
>
> Sure you can. Just make sure you're using a non-strict state monad.

Fair enough. I over-simplified the example I had in mind, which cannot 
be evaded so easily.

Though it's worth pointing out that your solution relies on the specific 
property I mentioned (later in the same email), namely that it is safe 
to perform the evaluation of lazy state at any point we desire, because 
it will not interact with other side-effects[1]. Thus, this happens to 
be one of the monads which has the property necessary for being able to 
perform the reordering desired by the OP. However, if we try using your 
'repeatM' for other monads it is unlikely to work--- and for the same 
reasons that 'many' and 'some' are problematic for Maybe and lists.


[1] Barring side-effects which perform introspection on the runtime 
system (e.g., to determine current memory usage, whether a thunk has 
been forced or not, etc.).

-- 
Live well,
~wren