Future of string lambda functions/string predicate functions

[Tyler Jameson Little]

On Sunday, 11 August 2013 at 16:26:16 UTC, Andrei Alexandrescu 
wrote:
> On 8/8/13 9:52 AM, Jonathan M Davis wrote:
>> On Thursday, August 08, 2013 07:29:56 H. S. Teoh wrote:
>>> Seems this thread has quietened down. So, what is the 
>>> conclusion? Seems
>>> like almost everyone concedes that silent deprecation is the 
>>> way to go.
>>> We still support string lambdas in the background, but in 
>>> public docs we
>>> promote the use of the new lambda syntax. Would that be a fair
>>> assessment of this discussion?
>>
>> I find it interesting that very few Phobos devs have weighed 
>> in on the matter,
>> but unfortunately, most of the posters who have weighed in do 
>> seem to be
>> against keeping them.
>
> There's a related issue that I think we must solve before 
> deciding whether or not we should deprecate string lambdas. 
> Consider:
>
> void main() {
>     import std.range;
>     SortedRange!(int[], "a > b") a;
>     SortedRange!(int[], "a > b") b;
>     b = a;
>     SortedRange!(int[], (a, b) => a > b) c;
>     SortedRange!(int[], (a, b) => a > b) d;
>     d = c;
> }
>
> The last line fails to compile because D does not currently 
> have a good notion of comparing lambdas for equality. In 
> contrast, string comparison is well defined, and although 
> string lambdas have clowny issues with e.g. "a>b" being 
> different from "a > b", people have a good understanding of 
> what to do to get code working.
>
> So I think we should come up with a good definition of what 
> comparing two function aliases means.
>
>
> Andrei

Correct me if I'm wrong, but AFAICT the old behavior was an 
undocumented feature. I couldn't find string lambdas formally 
documented anywhere, but lambdas are.

Comparing function aliases is an optimization, not a feature, so 
I don't feel it's a blocker to deprecating string lambdas. If the 
user needs the old behavior, he/she can do this today with an 
actual function:

     bool gt(int a, int b) {
         return a > b;
     }

     void main() {
         import std.range;
         SortedRange!(int[], "a > b") a;
         SortedRange!(int[], "a > b") b;
         b = a;
         SortedRange!(int[], gt) c;
         SortedRange!(int[], gt) d;
         d = c;
     }

While not as concise, this is safer and does not rely on 
undocumented behavior.

Another consideration, are the following equivalent?

     (a,b) => a > b
     (b,c) => b > c