Re: OO versus RDB

Marshall wrote:
> Heh. Funny you should mention that. It's also the case that
> one can have a function that writes to global variables
> but doesn't *read* from them, and it will *still* be
> pure, in that the outputs depend only on the inputs.
>
> The functional guys *really* hate it when I say *that*.

Well maybe that's because you're wrong :-)

Such a "function" isn't a function at all, and common program transformations and compiler optimisations (e.g. common sub-expression elimination) will modify/break programs in such cases.

Basically, in order to be considered "pure" it is necessary but not sufficient that the result delivered by a function is dependent only on the values of it's arguments. It's also necessary that evaluation has no other observable effect. So a programs observable behavior is identical whether the same expression is evaluated just once or several times (or maybe not at all) and is also independent of evaluation order.

Regards

--
Adrian Hey