Re: [Caml-list] Purity and lazyness

[Florian Weimer <fw@deneb.enyo.de>]

* Elias Gabriel Amaral da Silva:

>> As specified, Haskell is not a pure language because every pattern
>> match can have side effects.  The Haskell community is split between
>> those who think that this is a good thing, and those that consider it
>> problematic.  (Obviously, there is a large pure subset, much more
>> useful than Erlang's pure subset and covering almost the whole
>> language; you just avoid lazy I/O and use unsafePerformIO only for
>> correcting the type of functions imported through FFI.)
>
> Wait, a pattern match can have side effects? Can you provide some
> example code? (do you mean, pattern match failure / exceptions / run
> time errors?)

<http://article.gmane.org/gmane.comp.lang.haskell.prime/3083>

It uses seq instead of pattern matches for clarity.  But laziness
means that you could hide the side effect in any pattern match
whatsoever.

> I'm new to Haskell, but in my understanding, it is said that Haskell
> is pure because the whole Haskell code is just specification of
> (types, .. and) values, and this specification is enjoy referential
> transparency.

A small part of the standard library pretends that some externally
visible effects are not impure.  Other programmers follow that style.
In their code bases, you will occasionally see commits which add
additional laziness to avoid blocking on network input (when the
client will never send data because the protocol is half-duplex) or to
avoid reading too much data from disk.