conv text and pure

[H. S. Teoh]

On Wed, Oct 23, 2013 at 11:17:30PM +0200, Daniel Davidson wrote:
> On Wednesday, 23 October 2013 at 20:18:39 UTC, Andrej Mitrovic
> wrote:
> >On 10/23/13, Daniel Davidson <nospam at spam.com> wrote:
> >>Great, thanks. What is the best way to get on that version for the
> >>Mac (pointer to instructions)?
> >
> >You can download the beta here:
> >http://forum.dlang.org/thread/52605C84.6010109@walterbright.com
> 
> Thanks for pointer. I am using it and the file conv.d has:
> 
> string text(T...)(T args) { return textImpl!string(args); }
> 
> 
> So, when you say it is pure, what are you referring to. I think pure
> could be specified as a block, but when I search up the next prior
> pure occurrence is:
>   @safe pure unittest

No, pure can't be specified as a block. You can only mark a function as
pure.

Furthermore, the above function is a template function, which means the
compiler will perform attribute inference on it. So provided textImpl
has no impure operations, it should be inferred as pure, unless args has
some impure methods in it that's being used by textImpl.


> So I don't think it is in a pure block. Also I still get:
> 
> Error: pure function 'plus.utils.history.History!(const(S), "a.date
> < b.date").History.opOpAssign!("~", S).opOpAssign' cannot call
> impure function 'std.conv.text!(string, string, const(S), string,
> const(S)).text'
> 
> So, I am still looking for workaround if possible and answers to the
> general questions on pure.
[...]

OK, I'm looking at the implementation of textImpl in std/conv.d, and it
seems that the only thing it does is to call to!string(...) on each of
its arguments, and append the result to a local variable called
'result'. Since array appending is pure (otherwise we have major
problems with D purity), the only culprit seems to be the calls to
to!string().

What arguments are you passing to text()? Do all of them have toString
methods that are marked pure? Keep in mind that the default toString
implementation (e.g. inherited from Object) may not necessarily be
marked pure, so that could be the cause of your problems here.

It would be nice if you could provide a suitably minimized version of
your code that exhibits this purity issue, so that we can track it down.
It's kinda hard to figure out what's wrong based on verbal descriptions
alone. (I apologize if you've already posted said code; I've been too
busy to keep up with every post in this forum.)


T

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