[Haskell-cafe] Lazy series [was : Preventing sharing]

Arjen arjenvanweelden at gmail.com
Sat Jan 9 18:04:17 UTC 2016

Sorry, forgot to reply to the list.

On 01/09/2016 06:36 PM, Tom Ellis wrote:
> On Sat, Jan 09, 2016 at 06:29:05PM +0100, Jerzy Karczmarczuk wrote:
>> Tom Ellis wrote :
>>> consider a lazy language, Haskell--,/which doesn't allow recursive bindings of non-function types./   In Haskell-- you
>>> *cannot*  write
>>>      exps = 1 + integral exps
>>> but you have to write
>>>      exps = I.fix (\e -> 1 + integral e)
>>> So we see that the nice syntax "exps = 1 + integral exps" is not due to
>>> laziness (since Haskell-- is lazy, but you cannot write that).
>> If you say so...
>> You may always say:
>> "Consider the syntax XXXX.  Now, consider a lazy language which
>> doesn't allow XXXX.
>>   So, your nice syntax has nothing to do with laziness. QED".
> Granted, but the more important point was the sketch of the strict language
> which *does* allow it.  You have conveniently failed to challenge me on
> any of the aspects of the very simple design.
>> Tom, construct such a language, and I might believe you.
> I remind you that Doug's original claim was "this won't work in a strict
> language", which he offered without proof, even a sketch of a proof.  I
> still hold the onus is on you (or him) to demonstrate it!

If I'm not mistaken, a strict language implies that arguments are 
evaluated before function calls.
To calculate exps, you need to add 1 to the result of the function call 
integral on argument exps. To evaluate that call, it first evaluates the 
arguments: exps. And so on... This causes a non-terminating calculation, 
I would expect.

IMHO, unless you add explicit laziness to a strict language, which some 
do but it requires some extra syntax, this cannot be done. I do believe 
Scheme or ML does have laziness annotations, and they show in the data 
types and function call syntax.

>> Also, I recall your former objection, that
>> *exps = 1 + integral exps*
>> should work "for lazy lists" in a strict language. Please, implement
>> it.  Since you would need *letrec* anyway, I suggest Scheme (say,
>> Racket). You will see what that implies. Compare the behaviour of
>> strict and lazy Racket.
> Maybe since Scheme and Racket are not typed things will go through there.  I
> shall have to look into it.  I don't know the languages.
> Tom

If you propose that a function (or anything with a function type) is 
implicitly lazy, then I think that you are describing a lazy/non-strict 

kind regards, Arjen

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