A Small Contribution to Phobos

[bearophile]

Meta:

> 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, " "));

I'd like something like this in Phobos, but I'd like it to have a 
better name. But in most (all?) cases what I want to put inside 
such perform is a printing function, so I have opened this:

http://d.puremagic.com/issues/show_bug.cgi?id=9882



> 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;

I's also like this in Phobos, for debugging purposes. But I'd 
like it to return nothing, so you are forced to use it only at 
the end of a chain.

(So I appreciate 2 of your 4 proposals. I have proposed both of 
them in D.learn time ago.)

---------------------

Brad Anderson:

> Andrei didn't care for the tap() you propose but loved the idea 
> of a tap() function that works like unix tee.

Something like this Python itertool?

def tee(iterable, n=2):
     it = iter(iterable)
     deques = [collections.deque() for i in range(n)]
     def gen(mydeque):
         while True:
             if not mydeque:             # when the local deque is 
empty
                 newval = next(it)       # fetch a new value and
                 for d in deques:        # load it to all the 
deques
                     d.append(newval)
             yield mydeque.popleft()
     return tuple(gen(d) for d in deques)


Bye,
bearophile