DIP32: Uniform tuple syntax

[Timon Gehr]

On 03/29/2013 04:32 PM, kenji hara wrote:
> 2013/3/29 Timon Gehr <timon.gehr at gmx.ch <mailto:timon.gehr at gmx.ch>>
>
>     Looks quite nice. I especially like the {a, b} => ... thing.
>
>     I think, however, that there are a handful serious flaws that need
>     to be addressed:
>
>     0 "Inside tuple literal, ; never appears."
>        {{;}}           // a tuple not matching your specification
>
>
> It will be parsed as:
> {     // tuple braces

It contains ';', therefore the DIP says it is a function literal brace.

>      {;}   // function literal braces
> }
>
>        {{if(foo()){}}} // a non-tuple matching your specification
>
>
> {    // tuple braces
>      {    // function literal braces

There is no ';', therefore the DIP says it is a tuple brace.

>          if (foo()){}   // "if" always appears in statement scope
>      }
> }
>

I know how it _should_ be. The DIP contradicts what you say.

>     1 "Note: Cannot swap values by tuple assignment."
>        IMO a no-go. The syntax is too accessible to introduce this kind of
>        pitfall.
>
>
> Allowing value swap in tuple assignment will make language complex. I
> can't agree with it.
>

Quite obviously it is the other way round. There will be a never-ending 
flood of d.D.learn posts on the topic.

>     2 "// Error: cannnot use $ inside a function literal"
>        That's a DMD-ism presumably stemming from laziness during "fixing" of
>        an ICE/wrong code bug or something. I'd hate to carry this over to
>        the spec. Don't rely on it. The disambiguation is arbitrary, but may
>        be necessary. (It's not like it is a case actually occurring in real
>        code.)
>
>     3 Unpacking / pattern matching is underspecified.
>        - Do patterns nest?
>
>
> I think it should be allowed.
>

ok.

>        - Which right-hand sides are allowed with which semantics?
>
>
> Whether it is a pattern or a tuple-literal, is distinguished by their
> appeared locations.
>

Obviously, but this statement is not related to my question.
Valid right-hand sides seem to be at least tuples and expanded tuples 
(sequences). Anything else?


>        - Which left-hand sides are allowed with which semantics?
>          eg, what about:
>            ref int foo() { ... }
>            { foo(), foo() } = {1, 2};
>
>
> It will be lowered to:
> // { foo(), foo() } = {1, 2};
> foo() = 1;
> foo() = 2;
>

ok.

>     4 There is no way to capture the part matched by "..."
>
>
> I think this should be allowed.
>
> auto {x, r...} = tup;
> // Lowered to:
> // auto x = tup[0];
> // auto r = tup[1..$]
>
> `...` is very consistent token for this purpose.
>

Questionable.

> template X(T...) {}
> alias x = X!(int, long);  // T captures {int, long}
>

Not really. T captures {int, long}.expand.

>     5 .expand (or similar) property is missing.
>
>
> Use tup[]. It is already exists.
>

Slicing obviously shouldn't auto-expand. It's a shortcoming of the 
Phobos tuple introduced because static slicing cannot be overloaded.

>     6 Relation to {a: 2, b: 3}-style struct literals not explained.
>
>
> I am skeptical of the necessity of tuple literal with named fields.
>

Sure, but you'd at least have to argue in the DIP that the parser can 
distinguish the two, and how.

>     7 Tuple unpacking for template parameters not mentioned.
>
>
>     Is there a migration path for Phobos tuples planned?
>
>     Eg. template Tuple(T...){ alias Tuple = {T}; }
>          (field spec parsing left out for illustration)
>
>...

You have not answered the last two points.