[Haskell-cafe] On the purity of Haskell

[Steve Horne]

Sorry for the delay. I've written a couple of long replies already, and 
both times when I'd finished deleting all the stupid stuff there was 
nothing left - it seems I'm so focussed on my own view, I'm struggling 
with anything else today. Maybe a third try...

On 28/12/2011 19:38, AUGER Cédric wrote:
> Le Wed, 28 Dec 2011 17:39:52 +0000,
> Steve Horne<sh006d3592 at blueyonder.co.uk>  a écrit :
>
>> This is just my view on whether Haskell is pure, being offered up for
>> criticism. I haven't seen this view explicitly articulated anywhere
>> before, but it does seem to be implicit in a lot of explanations - in
>> particular the description of Monads in SBCs "Tackling the Awkward
>> Squad". I'm entirely focused on the IO monad here, but aware that
>> it's just one concrete case of an abstraction.

> IO monad doesn't make the language impure for me, since you can give
> another implementation which is perfectly pure and which has the same
> behaviour (although completely unrealistic):

> Now how would this work?
> In a first time, you load all your system file before running the
> program (a "side-effect" which does not modify already used structures;
> it is just initialization), then you run the program in a perfectly
> pure way, and at the end you commit all to the system file (so you
> modify structures the running program won't access as it has
> terminated).
I don't see how interactivity fits that model. If a user provides input 
in response to an on-screen prompt, you can't do all the input at the 
start (before the prompt is delayed) and you can't do all the output at 
the end.

Other than that, I'm OK with that. In fact if you're writing a compiler 
that way, it seems fine - you can certainly delay output of the 
generated object code until the end of the compilation, and the input 
done at the start of the compilation (source files) is separate from the 
run-time prompt-and-user-input thing.

See - I told you I'm having trouble seeing things in terms of someone 
elses model - I'm back to my current obsession again here.
> In Haskell,
> 'hGetChar h>>= \c ->  hPutChar i' always has the same value, but
> 'trans (hGetChar h>>= \c ->  hPutChar i) (IO_ A)'
> 'trans (hGetChar h>>= \c ->  hPutChar i) (IO_ B)'
> may have different values according to A and B.
>
> In C, you cannot express this distinction, since you only have:
> 'read(h,&c, 1); write(i,&c, 1);' and cannot pass explicitely the
> environment.
Agreed. Haskell is definitely more powerful in that sense.