[Issue 3395] Ambiguous array operations

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http://d.puremagic.com/issues/show_bug.cgi?id=3395



--- Comment #4 from Stewart Gordon <smjg at iname.com> 2011-11-30 03:02:18 PST ---
(In reply to comment #3)
> (In reply to comment #2)
> > (In reply to comment #0)
> > > These expressions are ambiguous:
> > > ---
> > > a[].max(n);
> > > a[1..4].max(n);
> > > ---
> > > Does it mean calling the function on the slice or on each item in the slice?
> > 
> > It means calling the function on the slice.  Unless I'm mistaken, there isn't
> > any D syntax at the moment that means calling the function on each element of
> > the array.
> 
> That's correct.
> 
> > > Possible solution is to change the meaning of empty square brackets from full
> > > slice to only a hint for array operation so that a[].max(n) is an array
> > > operation and a[1..4].max(n) is max(a[1..4],n).
> 
> > This would get confusing.  You might want to apply a function to the whole
> > slice [1..4] or to each element of the slice.  This applies whether the
> > array-property sugar is being used or not.
> > 
> > Perhaps the best solution is to define [] applied to the function identifier
> > itself to do an elementwise application.
> > 
> > So max(a, n) or a.max(n) would just call max(a, n) once.
> > And max[](a, n) or a.max[](n) would evaluate to an array of max(a[i], n).
> > And the same if a is replaced with a[], a[1..4] or some such in each case.
> 
> That looks to me as if max is an array of some struct S which defines an
> opCall.
> 
> > Of course, ambiguities can still occur in functions with multiple array
> > parameters.  Presumably the language would forbid it in these ambiguous cases,
> > as it does already with ambiguous overload matching.
> 
> Consider the case where we want y to be 
> [ max(x[2][0..$], max(x[3][0..$], ... ]
> 
> double [][20] x;
> double [10] y;
> 
> Brainstorming a few possibilities:
> 
>  y[] = max(x[2..12]);     // (1) looks like scalar assignment
>  y[] = max[2..12](x);     // (2)
>  y[] = max(x[2..12])[];   // (3)

That's ambiguous - maybe max is a function that returns an array or other type
with an opSlice().

> Can we put the [] _before_ the call? y[] = [] max(x);
> y[] = x.[]max;

Would [](expr) be the empty array's opCall(expr) or the vectorisation of the
function referenced by expr?  And [].func be a vectorisation of the global
function func or the empty array's .func method?  (Are you envisaging that []
vectorises a whole subexpression or just the function whose name it immediately
precedes?)

FWIW the other week I discovered C++11 variadic templates.  I wonder if we can
draw inspiration from the unpacking syntax here....
http://lanzkron.wordpress.com/2011/11/05/did-you-pack-that-yourself/

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