December 22, 2012
I sometimes write code like:

auto data = File("data.txt")
            .byLine()
            .map!(r => r.strip().dup)()
            .filter!(r => !r.empty)()
            .array();


To help writing/improving/debugging such code maybe it's useful to create a pass() range in Phobos. It's like a "identity range" that just passes its inputs to its output, but also runs a given lambda to the items that pass through it (the lambda receives a const item, to avoid modifications of the data passing through the chain):


auto data = File("data.txt")
            .byLine()
            .pass(r => writeln("A: ", r))
            .map!(r => r.strip().dup)()
            .pass(r => writeln("B: ", r))
            .filter!(r => !r.empty)()
            .array();


This usage of pass() allows to introduce debug prints inside very long chains of such iterables, with no need to break them apart and assign the parts to temporary variables just to see what's going on in that chain.

But it's not hard to do it with a not pure map:

                .map!((r){ writeln("B: ", r); return r; })()

Or even with an evil comma operator:

                .map!(r => (writeln("B: ", r), r))()

So I don't know how much useful that pass() is.

Bye,
bearophile
December 22, 2012
On Saturday, 22 December 2012 at 08:55:35 UTC, bearophile wrote:
> I sometimes write code like:
>
> auto data = File("data.txt")
>             .byLine()
>             .map!(r => r.strip().dup)()
>             .filter!(r => !r.empty)()
>             .array();
>
>
> To help writing/improving/debugging such code maybe it's useful to create a pass() range in Phobos. It's like a "identity range" that just passes its inputs to its output, but also runs a given lambda to the items that pass through it (the lambda receives a const item, to avoid modifications of the data passing through the chain):
>
>
> auto data = File("data.txt")
>             .byLine()
>             .pass(r => writeln("A: ", r))
>             .map!(r => r.strip().dup)()
>             .pass(r => writeln("B: ", r))
>             .filter!(r => !r.empty)()
>             .array();
>
>
> This usage of pass() allows to introduce debug prints inside very long chains of such iterables, with no need to break them apart and assign the parts to temporary variables just to see what's going on in that chain.
>
> But it's not hard to do it with a not pure map:
>
>                 .map!((r){ writeln("B: ", r); return r; })()
>
> Or even with an evil comma operator:
>
>                 .map!(r => (writeln("B: ", r), r))()
>
> So I don't know how much useful that pass() is.
>
> Bye,
> bearophile

Bug reporting aside, it can be useful in the sense of *doing* something to each element, as opposed to "call a function and re-assign to original element".

This would be especially true for say: Mutating functions that return void, or just plain calling member functions.

I think it's a legit request. If anything, *map* could be re-implemented in terms of *call*.
December 22, 2012
See also:

http://tech.blinemedical.com/debugging-piped-sequences-f/

Bye,
bearophile
December 22, 2012
On 12/22/2012 12:55 AM, bearophile wrote:

> To help writing/improving/debugging such code maybe it's useful to
> create a pass() range in Phobos.

I liked the idea yesterday but I thought that the name was a little off. How about a name like tap()? We tap into data as it's flowing through?

Ali

December 24, 2012
On Saturday, 22 December 2012 at 21:21:21 UTC, Ali Çehreli wrote:
> On 12/22/2012 12:55 AM, bearophile wrote:
>
> > To help writing/improving/debugging such code maybe it's
> useful to
> > create a pass() range in Phobos.
>
> I liked the idea yesterday but I thought that the name was a little off.

Ditto.

> How about a name like tap()? We tap into data as it's flowing through?

Niiice. I like it.

January 02, 2013
On Mon, Dec 24, 2012 at 02:35:29PM +0100, monarch_dodra wrote:
> On Saturday, 22 December 2012 at 21:21:21 UTC, Ali Çehreli wrote:
> >On 12/22/2012 12:55 AM, bearophile wrote:
> >
> >> To help writing/improving/debugging such code maybe it's
> >> useful to create a pass() range in Phobos.
> >
> >I liked the idea yesterday but I thought that the name was a little off.
> 
> Ditto.
> 
> >How about a name like tap()? We tap into data as it's flowing
> >through?
> 
> Niiice. I like it.

This response is a year late ;-) but I thought tee() might be a better name. It has a precedent in the Unix tee command, which copies stdin into a file and also passes it along to stdout, quite similar to what is being done here (except that instead of a file it's a delegate/function).


T

-- 
To err is human; to forgive is not our policy. -- Samuel Adler
January 02, 2013
H. S. Teoh:

> This response is a year late ;-)

No problem.


> but I thought tee() might be a better name.

Python programmers have this "tee":

http://docs.python.org/3/library/itertools.html#itertools.tee

Bye,
bearophile
January 02, 2013
On Wed, Jan 02, 2013 at 07:19:31PM +0100, bearophile wrote:
> H. S. Teoh:
[...]
> >but I thought tee() might be a better name.
> 
> Python programmers have this "tee":
> 
> http://docs.python.org/3/library/itertools.html#itertools.tee
[...]

Hmm. But isn't that just the same as repeatedly calling .save with D's forward ranges?


T

-- 
If you want to solve a problem, you need to address its root cause, not just its symptoms. Otherwise it's like treating cancer with Tylenol...
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