How do you use templates in D?

[Jonathan M Davis]

On Tuesday, October 18, 2011 15:37 Ali Çehreli wrote:
> On Tue, 18 Oct 2011 17:56:33 -0400, Jonathan M Davis wrote:
> > On Tuesday, October 18, 2011 12:32 Ali Çehreli wrote:
> >> On Tue, 18 Oct 2011 13:56:09 -0400, Andrew Pennebaker wrote:
> >> > I'm writing a function called genArray that accepts a function gen
> >> > and returns a random array populated by calling gen(). genString
> >> > builds on this by returning genArray(&genChar). But my type
> >> > signatures are wrong.
> >> > 
> >> > In dashcheck.d:
> >> > 
> >> > char genChar() {
> >> > 
> >> > return cast(char) uniform(0, 128);
> >> 
> >> It's a good thing that you are staying within ASCII. Otherwise there
> >> might be invalid UTF-8 sequences. :)
> > 
> > Yeah. It's generally a very bad sign when code uses char or wchar
> > outside of arrays. They're UTF-8 and UTF-16 code units respectively and
> > are not necessarily full code points (and therefore are not necessarily
> > full characters). That's particularly true of char, since anything
> > outside of ASCII is not going to work. Code which operates on individual
> > characters really should be operating on dchars, not chars or wchars.
> 
> Good tips, but we can further qualify "Code which operates on individual
> characters". I think you imply accessing individual characters, likely
> with the [] operator.
> 
> It is perfectly fine to use char and wchar strings as ranges when
> std.array is imported (it is automatically imported by std.range too):
> 
> import std.stdio;
> import std.range;
> import std.array; // <-- empowers (en-ranges :p) char and wchar strings
> 
> void main()
> {
> writeln(take("abcçdeé", 5));
> }
> 
> Outputs 5 Unicode characters, not code units.
> 
> In other words, e.g. .front on any type of string produces a dchar; and
> that may very well prove your point. ;)

Yes. When treating arrays of char and wchar as ranges, you're effectively 
operating on dchars, so you're fine. The problem is when you have a variable 
which is char or wchar or when youyou access an individual char or wchar in an 
array. Operating on chars and wchars is fine as long as you decode them to 
dchar when you do it (as happens when using range operations such as front), 
but operating on individual chars or wchars is almost always a bug.

- Jonathan M Davis