Re: [Caml-list] Purity and lazyness

[Elias Gabriel Amaral da Silva <tolkiendili@gmail.com>]

2011/1/7 Florian Weimer <fw@deneb.enyo.de>:
> * Dario Teixeira:
>
>> So, my question is whether there is something I'm missing and in fact "purity
>> <=> lazyness", or I am reading too much from those Haskeller presentations,
>> because they never meant to say anything beyond "lazyness => purity", and
>> freely mixing the two was just a casual oversight.
>
> 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?)

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.

One defines "main" to hold a certain value, which describe the
computations that will performed at run time; but main is defined
using compositions that preserve referential transparency (a >>= b has
value a that depends only on a and b; and in general, f a b .. has a
value that depends only on f, a, b, ..; I know no exception for this)