A Small Contribution to Phobos
Brad Anderson
eco at gnuk.net
Sat Jun 1 22:23:10 PDT 2013
On Sunday, 2 June 2013 at 02:57:56 UTC, Meta wrote:
> I saw a thread a few days ago about somebody wanting a few
> UFCS-based convenience functions, so I thought that I'd take
> the opportunity to make a small contribution to Phobos.
> Currently I have four small functions: each, exhaust, perform,
> and tap, and would like some feedback.
>
> each is designed to perform operations with side-effects on
> each range element. To actually change the elements of the
> range, each element must be accepted by reference.
>
> Range each(alias fun, Range)(Range r)
> if (isInputRange!(Unqual!Range))
> {
> alias unaryFun!fun _fun;
> foreach (ref e; r)
> {
> fun(e);
> }
>
> return r;
> }
>
> //Prints [-1, 0, 1]
> [1, 2, 3].each!((ref i) => i -= 2).writeln;
>
>
> exhaust iterates a range until it is exhausted. It also has the
> nice feature that if range.front is callable, exhaust will call
> it upon each iteration.
>
> Range exhaust(Range)(Range r)
> if (isInputRange!(Unqual!Range))
> {
>
> while (!r.empty)
> {
> r.front();
> r.popFront();
> }
>
> return r;
> }
>
> //Writes "www.dlang.org". x is an empty MapResult range.
> auto x = "www.dlang.org"
> .map!((c) { c.write; return false; })
> .exhaust;
>
> //Prints []
> [1, 2, 3].exhaust.writeln;
>
>
> perform is pretty badly named, but I couldn't come up with a
> better one. It can be inserted in a UFCS chain and perform some
> operation with side-effects. It doesn't alter its argument,
> just returns it for the next function in the chain.
>
> T perform(alias dg, T)(ref T val)
> {
> dg();
>
> return val;
> }
>
> //Prints "Mapped: 2 4"
> [1, 2, 3, 4, 5]
> .filter!(n => n < 3)
> .map!(n => n * n)
> .perform!({write("Mapped: ");})
> .each!(n => write(n, " "));
>
>
> Lastly is tap, which takes a value and performs some mutating
> operation on it. It then returns the value.
>
> T tap(alias dg, T)(auto ref T val)
> {
> dg(val);
>
> return val;
> }
>
> class Foo
> {
> int x;
> int y;
> }
>
> auto f = (new Foo).tap!((f)
> {
> f.x = 2;
> f.y = 3;
> });
>
> //Prints 2 3
> writeln(f.x, " ", f.y);
>
> struct Foo2
> {
> int x;
> int y;
> }
>
> //Need to use ref for value types
> auto f2 = Foo2().tap!((ref f)
> {
> f.x = 3;
> f.y = 2;
> });
>
> //Prints 3 2
> writeln(f2.x, " ", f2.y);
>
>
> Do you think these small functions have a place in Phobos? I
> think each and exhaust would be best put into std.range, but
> I'm not quite sure where perform and tap should go. Also,
> there's that horrible name for perform, for which I would like
> to come up with a better name.
You may find this forum discussion from several months ago
interesting.
http://forum.dlang.org/post/kglo9d$rjf$1@digitalmars.com
Confusingly, your each() seems to be fairly similar to what
Andrei wanted tap() used for. Andrei didn't care for the tap()
you propose but loved the idea of a tap() function that works
like unix tee.
I like exhaust() as I just had to write something similar. I
like perform() just because I love UFCS range chains and anything
to avoid those extra statements is alright in my book. This is
probably not a majority opinion though. I can't think of a
better name either though.
More information about the Digitalmars-d
mailing list