[Haskell-cafe] On the purity of Haskell

[Artyom Kazak]

Chris Smith <cdsmith at gmail.com> писал(а) в своём письме Fri, 30 Dec 2011  
22:28:36 +0200:

I really think that the notion of “purity” appeared to convince C  
programmers. It would be silly to try to explain that “Int -> IO Int”  
isn't really a function from Int to Int, monads, blah blah blah. So, we're  
saying: “here is a function which accepts Int and (sic!) returns Int, but  
it can also do IO, which is kinda unsafe. So we are marking it with ‘IO’  
letters, see? This means that this function is ‘impure’. And to prevent  
mixing pure things with impure things, compiler demands you to mark with  
IO every “function” which ‘uses’ IO”.

Functions don't use IO. Funtions return IO <something>. And functions  
returning IO are impure. That's all.

> This is a valid concern... assigning a meaning to values of IO types
> necessarily involves some very unsatisfying hand-waving about
> indeterminacy, since for example IO actions can distinguish between
> bottoms that are considered equivalent in the denotational semantics of
> pure values (you can catch a use of 'error', but you can't catch
> non-termination).  Nevertheless, I'm satisfied that to the extent that
> any such meaning can be assigned, f will be a valid function on
> non-bottom values.  Not perfect, but close.

Agree. The fact that IO actions can distinguish between bottoms,  
self-modify code, terminate non-terminable computations by rebooting the  
system, send killbots to the programmer's house and so on are extremely  
unsatisfying. That's IO for you.

The dirty impure bottom comparison which uses IO, though, is available  
only to already impure functions.