funny bug with input range interface and toString

[spir]

On Mon, 13 Dec 2010 17:37:19 +0300
"Nick Voronin" <elfy.nv at gmail.com> wrote:

> On Mon, 13 Dec 2010 11:24:48 +0300, spir <denis.spir at gmail.com> wrote:
> 
> > I have a strange bug with an input range interface. Initially, I had a  
> > (rather big) struct called Text with loads of working unittests. When  
> > adding a range interface, noting worked anymore, any test ran into an  
> > infinite loop (the terminal writes pages of '[') ending with segfault.
> 
>      @property Text front () {
>          return Text(this.stacks[this.rangeIndex]);
>      }
> 
> Um. There is recursion. front() of Text returns Text. formatValue iterates  
> through Range, trying to format every element of it... but goes nowhere  
> because Range.front never yields anything but itself typewise.

Yes, but returning an element of the same type is the write thing here. (Like in dynamic languages where s[i] returns a singleton s).

> > After shrinking the bug's scope,I finally found it happens on simple  
> > writeln(text).
> 
> Not so simple, with all underlying mess of templates :-D I suppose it's a  
> bug that instantiation of formatValue for Range types there always tries  
> to iterate manually, ignoring existing toString.
> 
> void formatValue(Writer, T, Char)(Writer w, T val,
>          ref FormatSpec!Char f)
> if ( ( is(T == struct) || is(T == union) ) && !isInputRange!T)
> 
> As you can see it even explicitly mention that default formatting for  
> struct (which can use toString) not to be used for Ranges. I think it's  
> wrong.

You are right. (With some offline help) finally ended up following the same reasoning and got to the same conclusion. This is clearly a bug (actualy 2 or 3). I'll post about it on D's main mailing list.

> > https://bitbucket.org/denispir/denispir-d/changeset/39200b499fb9#chg-textbug.d.  
> > Note that the range interface itself works fine.
> 
> btw, I wonder, how do you build it? I ended up with very long command line  
> for dmd, as there was no makefile and jake/rdmd failed to produce anything  
> ether.
I use rdmd for quick testing (because it links automagically).
compile: dmd -w -c filename.d
build: rdmd -w -offilename" -debug -unittest --build-only filename.d

> > So, it seems that, in my case for any reason, introducing an input range  
> > lets D shortcut toString (and try to use what instead?). Look like a bug?
> 
> Yup! Still, stack overflow is due to unconventional Range interface.

Yes. But the formatValue template selection criteria _must_ check this!

Denis
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