[Issue 2832] pure function too pure

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http://d.puremagic.com/issues/show_bug.cgi?id=2832



--- Comment #4 from Max Samukha <samukha at voliacable.com> 2009-10-13 08:44:23 PDT ---
> I think the existing behaviour -- that you cannot change any of the parameters
> in a pure function -- is simple and intuitive: pure functions can only modify
> variables which they created themselves.

But the passed-by-value parts of the arguments are copied and consequently can
be qualified as "variables which they created themselves".

> A rule that pure nested functions can
> use indirectly-referenced data, but cannot use parameters which are passed by
> value, just seems complicated.

I don't think it is too complicated. It can be trivially done like this:

pure int foo(in int d, ...)
{
 // now we should be able to use d in pure nested functions because it is 
 // guaranteed to not change during the function call.
}

> Especially, in the case where a parameter contains a reference to other data,
> it seems folly to be allowed to change part of the parameter, but not all of
> it.

I am not sure. For example, it seems to be fairly intuitive to be able to
rebind a string parameter, though changing the referenced part of it is not
allowed. I would agree if D's function parameters behaved like aliases to the
arguments, but they are more like the function's local variables, which
arguments are assigned to.

Now that I am trying to purify some functions (most of which have no nested
functions) I need to add the useless temporaries to make them compile :(

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