Why I'm Excited about D

[Walter Bright via Digitalmars-d]

On 4/8/2015 10:33 AM, Adam D. Ruppe wrote:
> On Wednesday, 8 April 2015 at 17:17:21 UTC, Walter Bright wrote:
>> writeln is an OutputRange.
>
> No, it isn't. It isn't even remotely close to one - an OutputRange is a type,

I meant an instance of a type, as in:

    i is an int

meaning i has type int.

> writeln is a function. isOutputRange doesn't even return false for it, it
> outright fails to compile!

I understand that a true OutputRange would require one to write:

    arg.copy(writeln);

and that a common idiom is to write it so the copy() can be implicit. I haven't 
made up my mind as to the right style here.


> File isn't an output range either btw. An output range is defined by the
> presence of a put method, which File doesn't have.
>
> b.d(4): Error: static assert  (isOutputRange!(File, char)) is false
>
>
> Even putting aside the formal definition of output range, output is NOT part of
> the pipeline! Pipelines can be composed and they keep pumping data through them.

An output range is the caboose of a pipeline (to mix metaphors).

> writeln does not transform its input nor does it forward it to the next item in
> the pipe. It just eats it.

It places its input into a container.

> That's totally different than everything else in the sequence. And that's not
> even bringing in writeln's sister function, writefln, whose first argument isn't
> what is to be printed at all!

It's true that a lot of Phobos code was designed pre-ranges, and the parameters 
aren't set up ideally for it. I've been doing some work to fix that.