I was doing some metaprogramming where I wanted to make a slice of a type:

alias Tbasic = int;
pragma(msg, Tbasic);
alias Tbasica = Tbasic[];
pragma(msg, Tbasica);

//prints int, int[]

And things worked fine until I attempted the same thing on what happened to be a tuple of 1 element:


alias Ts = AliasSeq!int;
pragma(msg, Ts);
alias Tsa = Ts[];
pragma(msg, Tsa);
//prints (int), (int)

which confused me until I realized that the [] was slicing the tuple.  Note, you can add as many [] as you want since it's basically a no-op.

pragma(msg, Ts[0][]); //prints int[]

Is there any use for this behavior?  It seems like it might be worth warning like "slicing a tuple is a no-op"

Anyway, figured I'd post here for posterity since I was confused by it for a while.

On 5/7/20 8:33 AM, Ben Jones wrote:> I was doing some metaprogramming where I wanted to make a slice of a type:

[...]

> alias Ts = AliasSeq!int;
> pragma(msg, Ts);
> alias Tsa = Ts[];
> pragma(msg, Tsa);
> //prints (int), (int)
>
> which confused me until I realized that the [] was slicing the tuple.
> Note, you can add as many [] as you want since it's basically a no-op.

[...]

> Is there any use for this behavior?

Slicing an alias sequence is definitely useful e.g. in recursive templates:

  alias car = T[0];      // (head)
  alias cdr = T[1..$];   // (tail)

> It seems like it might be worth
> warning like "slicing a tuple is a no-op"

The trouble seems to be when slicing the entire tuple. Even in that case, printing a warning would not be desired in some situations ironically in generic code where e.g. T[0..$] may appear, which is the same as T[].

Ali

On Thursday, 7 May 2020 at 23:07:40 UTC, Ali Çehreli wrote:

> The trouble seems to be when slicing the entire tuple. Even in that case, printing a warning would not be desired in some situations ironically in generic code where e.g. T[0..$] may appear, which is the same as T[].
>
> Ali

I agree T[0..$] and T[] mean the same thing and the first could come up naturally, but if there's never a useful cased spelled with T[], then it would be detectable and nice to issue a warning.