Re: [Caml-list] Purity and lazyness

["Holger Weiß" <holger@cis.fu-berlin.de>]

* Dario Teixeira <darioteixeira@yahoo.com> [2011-01-07 07:35]:
> In presentations by Haskellers, lazyness and purity are often portrayed as
> going hand in hand.  Now, I can see why a language which is lazy by default
> would also need to be pure, since side-effects would be indeed very messy 
> if evaluation order is not predictable.  However, I cannot see the converse,
> that is, I don't see why purity would require lazyness. 
> 
> So, my question is whether there is something I'm missing and in fact "purity 
> <=> lazyness", or I am reading too much from those Haskeller presentations,
> because they never meant to say anything beyond "lazyness => purity", and
> freely mixing the two was just a casual oversight.

Simon Peyton-Jones argues like this:

| Because Haskell is lazy it meant that we were much more consistent about
| keeping the language pure.  You could have a pure, strict, call by value
| language, but no one has managed to do that because the moment you have
| a strict call by value language, the temptation to add impurities (side
| effects) is overwhelming.  So "laziness kept us pure" is the slogan!

[ http://www.techworld.com.au/article/261007/a-z_programming_languages_haskell/?pp=7 ]

Holger