Re: [PATCH for Dlang support 1/2] d: create the Parser.Context class

[Akim Demaille]

Hi H.S.,

> Le 27 oct. 2020 à 17:55, H. S. Teoh <hsteoh@quickfur.ath.cx> a écrit :
> 
> On Tue, Oct 27, 2020 at 07:38:46AM +0100, Akim Demaille wrote:
> [...]
>>> +  /**
>>> +   * Information needed to get the list of expected tokens and to forge
>>> +   * a syntax error diagnostic.
>>> +   */
>>> +  public static final class Context
>>> +  {
>>> +
>>> +    private YYStack yystack;
>> 
>> Is this a copy of the stack?  It does not look like a
>> reference/pointer to the this.  We must not copy it, it's big, and
>> readonly here.  It should be "const", but I don't know how that is
>> spelled in D.
> [...]
> 
> Is YYStack a struct or class? Or an array? If it's an array or class,
> it's a reference type by default.

It's a struct.  But I guess it's not so bad as it contains only an
array:

  private final struct YYStack {
    private YYStackElement[] stack = [];
  ...

so after all, it is not so big, I expect the inner array to be
"copied" by reference.


> W.r.t. const, there are several ways to spell it, depending on the
> intended semantics.  If it should be an immutable reference, then you
> could write it as:
> 
>       private const(YYStack) yystack;
> 
> In this case, the variable cannot be reassigned after initialization,
> and if it's a reference type, it cannot be rebound.  Also, any elements
> you retrieve from it will be const (const in D is transitive).

I believe this is what we need here.

> If it's an array, you can optionally have a mutable slice of immutable
> elements (i.e., a view that can change, but the underlying elements are
> not changeable):
> 
>       private const(E)[] yystack;     // where E is the element type of 
> YYStack

Now I see why const takes parens.

Thanks a lot for your explanations!