Future of string lambda functions/string predicate functions

[H. S. Teoh]

On Wed, Aug 14, 2013 at 07:03:32PM +0200, John Colvin wrote:
> On Wednesday, 14 August 2013 at 16:38:57 UTC, H. S. Teoh wrote:
> >On Wed, Aug 14, 2013 at 09:26:20AM -0700, Andrei Alexandrescu
> >wrote:
> >>On 8/14/13 7:34 AM, H. S. Teoh wrote:
> >[...]
> >>>That's a bit too terse. What about this:
> >>>
> >>>	less		// a < b
> >>>	less!(5)	// a < 5
> >>>	lessEq		// a <= b
> >>>	lessEq!(5)	// a <= 5
> >>>	more		// a > b
> >>>	more!(5)	// a > 5
> >>>	moreEq		// a >= b
> >>>	moreEq!(5)	// a >= 5
> >>>	equal		// a == b
> >>>	equal!(5)	// a == 5
> >>>	notEqual	// a != b
> >>>	notEqual!(5)	// a != 5
> >>
> >>At this point using "a < b" for a < b, "a < 5" for a < 5 etc.
> >>becomes awfully attractive.
> >[...]
> >
> >It just occurred to me, that perhaps what we really need here is an
> >even more abbreviated form of lambda literals, like this:
> >
> >	sort!(a < b)(range);
> >
> >where 'a' and 'b' are undefined identifiers in the current scope, and
> >the compiler would know to bind them to lambda parameters. Defined
> >identifiers would, naturally, bind to whatever they refer to:
> >
> >	int x = 5;
> >	find!(a == x)(range);
> >
> >This would be equivalent to:
> >
> >	int x = 5;
> >	find!((a) => a==x)(range);
> >
> >IOW, an expression that references undefined identifiers in a
> >template parameter would be turned into a lambda that parametrize
> >said identifiers.
> >
> >
> >T
> 
> That's a bit too fragile IMO.
> 
> int b; //rename to 'a'
> // lots of intermediate code...
> int x = 5;
> r.blah!(a == x);
> 
> Renaming of a seemingly unrelated variable would result in spooky
> action-at-a-distance behaviour.

Arguably, naming a variable as 'b' or 'a' is a bad idea, but, point
taken.

I guess we still have to fall back to full-fledged lambda literals,
then. I'm still in favor of deprecating string lambdas, but as Andrei
said, we need to address the problem of how to compare lambdas
meaningfully.

The question then becomes, what do we do *now*, for new additions to
Phobos? Should new code still support string lambdas or not?


T

-- 
Right now I'm having amnesia and deja vu at the same time. I think I've forgotten this before.