[Haskell-cafe] Re: Why?

[MightyByte]

On Thu, Dec 10, 2009 at 11:56 AM, Sebastian Sylvan
<sebastian.sylvan at gmail.com> wrote:
>
>
> On Thu, Dec 10, 2009 at 3:30 PM, John D. Earle <JohnDEarle at cox.net> wrote:
>>
>> My intuition says that laziness and purity are distinct whereas yours says
>> that purity is a necessary condition. This is what needs to be reconciled.
>
> I think laziness requires purity to make sense. Laziness implies that the
> order of evaluation is highly unpredictable and depends strongly on the
> implementation details of libraries and such (which you may not have access
> to). So it's fickle. Someone adds an if statement somewhere and all of a
> sudden a variable gets evaluated earlier than it used to. It would be
> madness to write any code which depends on this unpredictable behaviour. In
> other words, the expressions that get evaluated lazily must not have side
> effects.

Yes, this is discussed in section 3.2 of the paper I cited earlier
(http://research.microsoft.com/en-us/um/people/simonpj/papers/history-of-haskell/history.pdf).
 That paper gives some nice insight into the history of Haskell.