[Haskell-cafe] Are constructors strict?

[Sebastian Fischer]

sorry, forgot to cc cafe.

On Fri, Jan 21, 2011 at 7:12 PM, Sebastian Fischer <fischer at nii.ac.jp>wrote:

> Hi Daryoush,
>
> On Fri, Jan 21, 2011 at 6:18 AM, Daryoush Mehrtash <dmehrtash at gmail.com>wrote:
>
>> I am having hard time understanding the following paragraph in "Purely
>> functional Lazy non-deterministic  programing" paper
>> http://www.cs.rutgers.edu/~ccshan/rational/lazy-nondet.pdf
>>
>> The problem with the naive monadic encoding of non-determinism is that the
>> arguments to a constructor must be deterministic. If these arguments are
>> themselves results of non-deterministic computations, these computations
>> must be performed completely before we can apply the constructor to build a
>> non-deterministic result.
>>
>>
>>
>> Why does the argument to constructors must be deterministic?
>>
>
> Because of the type of the constructor. Say you want to compute a list of
> Ints nondeterministically (like in the paper). Than the first argument of
> the (:) constructurs in the result must be an Int. A nondeterministic
> computation that yields an Int has type (m Int) for some nondeterminism
> monad (for example, it has type [Int] in the list monad). That the (:)
> constructur is polymorphic and would allow to make a value of type [m Int],
> is a coincidence and does not help to make a value of type (m [Int]).
>
>
>> WHy is it that thunks are not used in this case?
>>
>
> Thunks do not model nondeterminism, only laziness.
>
> Did that help you understand?
>
> Regards,
> Sebastian
>
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