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Date: Fri, 7 Jan 2011 20:32:13 +0100
From: Petter Urkedal <paurkedal@gmail.com>
To: caml-list@inria.fr
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Subject: Re: [Caml-list] Purity and lazyness

On 2011-01-07, Brian Hurt wrote:
> 
> 
> On Fri, 7 Jan 2011, Holger Weiß wrote:
> 
> > 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 ]
> >
> 
> Unless there is some other driver to keep things pure even while being 
> strict.  And I would argue there is- concurrency.  Concurrency has a lot 
> of similarities with laziness, in that the ordering of computations can be 
> (and often is) undefined, with all the fun that entails.  Haskell is 
> really good at multithreaded because it has already "paid the price" of 
> dealing with asynchronous computations.

And on the more theoretical side, strictness in not sufficient to make
impurity sound in ML, since the let-polymorphism must also be
restricted, e.g. with the value-restriction.