missing HexString documentation

[Steven Schveighoffer]

On 2/8/18 1:25 PM, Walter Bright wrote:
> On 2/8/2018 7:07 AM, Steven Schveighoffer wrote:
>> My concern in the hexString case is the sheer requirement of CTFE for 
>> something that is so easy to do in the compiler, already *done* in the 
>> compiler, and has another form specifically for hex strings (the 
>> "\xde\xad\xbe\xef" form) that isn't going away. It makes me laugh 
>> actually that Walter is now replacing the implementation with a mixin 
>> of that other form, incurring all the cost of CTFE so you can 
>> transform the string, while breaking existing code in the process: 
>> https://github.com/dlang/phobos/pull/6138
> 
> The breakage was due to the original implementation of hexString not 
> producing a string literal like "abc", but producing an array literal 
> like ['a', 'b', 'c'], which was not what the documentation said it did. 
> And naturally, some uses wound up relying on the array behavior.

"abc" is an array (it's an immutable(char)[]). There's no reason why 
['a','b','c'] should be different than "abc" (other than the hidden null 
character, which is irrelevant here).

Perhaps the fact that using a string rather than an array causes code to 
fail should be addressed?

> 
> What the PR does is fix hexString so that hexString!"deadbeef" rewrites 
> it to the string literal "\xde\xad\xbe\xef". It's classic "lowering". 
> Isn't it amazing that D can even do this?

It's great that D has this power, and would be really useful if D's 
language didn't already have a way to do this in a builtin way.

> Simplifying the compiler and pushing things off into the library makes 
> the compiler and spec smaller and less buggy. It also has the nice 
> feature of providing a simple path for anyone who wants to write their 
> own custom string syntax, such as EBCDIC string literals (!).

How can this be a huge simplification? I mean you already have code that 
parses hex characters in a string array, all you need is one flag that 
assumes all character pairs have been preceded by \x. I think this will 
save probably 4 or 5 lines of code?

It also doesn't preclude at all someone writing library code to make 
their own custom string syntax.

-Steve