[Haskell-cafe] On the purity of Haskell

[Jerzy Karczmarczuk]

Sorry, a long and pseudo-philosophical treatise. Trash it before reading.

Heinrich Apfelmus:
> You could say that side effects have been moved from functions to some 
> other type (namely IO) in Haskell. 
I have no reason to be categorical, but I believe that calling the 
interaction of a Haskell programme with the World - a "side effect" is 
sinful, and it is a source of semantical trouble.

People do it, SPJ (cited by S. Horne) did it as well, and this is too bad.
People, when you eat a sandwich: are you doing "side effects"??  If you 
break a tooth on it, this IS a side effect, but neither the eating nor 
digesting it, seems to be one.

This term should be used in a way compatible with its original meaning, 
that something happens implicitly, "behind the curtain", specified most 
often in an informal way (not always deserving to be called 
"operational"). If you call all the assignments "side effects", why not 
call -  let x = whatever in Something - also a "local side-effect"?
Oh, that you can often transform let in the application of lambda, thus 
purely functional?

Doesn't matter, Steve Horne will explain you that (sorry for the irony): 
"let is a compile-time pure construct ; at execution this is impure, 
because x got a value".

S.H. admits that he reasons within his model, and has problems with 
others. Everybody has such problems, but I see here something the (true) 
Frenchies call "un dialogue de sourds". For me a Haskell programme is 
ABSOLUTELY pure, including the IO. The issue is that `bind` within the 
IO monad has an implicit parameter, the World. In fact, a stream of 
Worlds, every putWhatever, getLine, etc. passes to a new instance.

We do not control this World, we call it "imperative" (whatever this 
means, concerning eating a sandwich, or exploding an impure neutron 
bomb), so we abuse the term "side effect" as hell!
The "Haskell sector" of the global world, the programme itself is just a 
function. Pure as the robe of an angel. Simply, you are not allowed by 
the Holy Scripts to look under this robe.

==

The rest is a (pure of course) délire.
Well, you might not believe me, but philosophically you don't need to 
imagine the World as imperative. Personally I am a believer in the 
Quantum Religion. If you accept all them Holy Dogmas of Unitarity, of 
Linearity, etc., if you believe in the True Quantum Nature of the real 
world, - then it becomes ... functional. Pure.
  Without a single trace of any "side effects".

The problem is that residing inside this world precludes the possibility 
of considering *observed things* as pure, they are conceptually detached 
from the stream of the Universe Vectors. They "change", so you say: 
HaHa!! A particle got ASSIGNED a new position! This is an imperative 
side-effect! -
- while from the point of view of an external observer, a common 
evolution operator transformed both of you, YOU and the particle into a 
new instance of this sector.

OK, I stop here, otherwise the digestion of your sandwiches may produce 
some side effects.

Jerzy Karczmarczuk
Caen, France.
(William the Conqueror was here. Produced one nice side-effect.)