On Mon, Mar 10, 2014 at 9:14 PM, Jonathan M Davis <jmdavisProg@gmx.com> wrote:
On Monday, March 10, 2014 21:50:25 Nick Sabalausky wrote:
> On 3/10/2014 9:24 PM, Timothee Cour wrote:
> > Is there a way to do the following lazily:
> >
> > writelnIfNotEmpty(T)(T a){
> > auto b=text(a);
> > if(b.length)
> >
> >    writeln(b);
> >
> > }
> >
> > ie, without using std.conv.text (which needlessly computes an intermediate
> > string, which could be quite large) or launching a separate process ?
> >
> > writelnIfNotEmpty(""); //doesn't print new line
> > writelnIfNotEmpty("a"); //prints "a\n"
>
> Sounds like what you need is a version of to!string() or text() that
> takes an output sink. Taking a look at std.conv, I'm kinda surprised I
> don't see one :/

std.format.formattedWrite will do the equivalent of writef to an output range,
though I'm not sure that that will really do what the OP wants, since it would
still have to write the result to an output range even if it were empty, and
odds are that the output range would be something on the heap anyway (e.g.
Appender is technically on the stack, but it's contents are on the heap),
making it so that it probably doesn't help much in this case.

Though to be honest, I'm not quite sure why writelnIfNotEmpty would be very
useful unless what's being passed in would result in the empty string, and I
would think that that would almost always be detectable (the one exception
being user-defined types whose toString results in an empty string). Something
as simple as

void writelnIfNotEmpty(T)(T a)
{
    static if(isInputRange!T)
    {
        if(!a.empty)
            writeln(a);
    }
    else
        writeln(a);
}

would then cover most cases - the one exception being toStrings which can
result in empty. And if that's a concern, then something like

    else static if(is(T == struct) || is(T == class))
    {
        auto b = to!string(a);
        if(b.length)
            writeln(b);
    }

should take care of the toString case. It doesn't avoid creating an
intermediate string, but unless the toString takes an output range, it's
always going to allocate anyway, and if it does take an output range, once
again, you'd need one which somehow avoided allocating altogether, which isn't
particularly likely.

Alternatively, you could just assume that no toString will result in the empty
string, as it's probably pretty rare that it would, but I'm not sure that that
would actually save you any overhead except in the case where the toString
takes an output range (since otherwise, it'll allocate a new string
regardless), but toStrings which take output ranges are fairly uncommon at
this point.

- Jonathan M Davis


Thanks,
that indeed works in many cases but there are still others (eg what if empty() prints, say in debug mode: writelnIfNotEmpty would not print the debug information even though writeln() would execute that debug code and print something).

I was wondering whether there would be a robust way that would check whether anything was written to stdout (eg via accessing raw file pointer/C lib), eg something like that :

void writelnIfNotEmpty(T)(T a){
auto file_pos=get_filepos(stdin);
write(a);
if(get_filepos(stdin)!=file_pos)
writeln;
}