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

           Summary: Add UFCS-friendly printing functions
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: Phobos
        AssignedTo: nobody@puremagic.com
        ReportedBy: bearophile_hugs@eml.cc


--- Comment #0 from bearophile_hugs@eml.cc 2013-04-05 05:27:08 PDT ---
This program prints the transposed of some strings. You can see that if you want to print the result of a long UFCS chain with a formatting string you are forced to break the chain, and it's not immediate to print intermediate steps:


import std.stdio: writeln, writefln;
import std.range: iota, transversal;
import std.algorithm: map, reduce, max;

void main() {
    immutable txt = ["Line one", "Line 2"];

    writefln("%(%s\n%)", txt
                         .map!q{ a.length }
                         .map!((s){ writeln(s); return s; })
                         .reduce!max
                         .iota
                         .map!(i => txt.transversal(i)));
}



Output:

8
6
LL
ii
nn
ee

o2
n
e

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

A simple way to solve both problems is to add to std.stdio four printing functions that are UFCS-friendly, where the format string is leading:


import std.stdio: write, writeln, writef, writefln;
import std.traits: isSomeString;

auto uWrite(T)(T data) {
    write(data);
    return data;
}

auto uWriteln(T)(T data) {
    writeln(data);
    return data;
}

auto uWritef(T, TS)(T data, TS format) if (isSomeString!TS) {
    writef(format, data);
    return data;
}

auto uWritefln(T, TS)(T data, TS format) if (isSomeString!TS) {
    writefln(format, data);
    return data;
}

// Usage demo -------------

import std.range: iota, transversal;
import std.algorithm: map, reduce, max;

void main() {
    immutable txt = ["Line one", "Line 2"];

    txt
    .map!q{ a.length }
    .uWriteln
    .reduce!max
    .iota
    .map!(i => txt.transversal(i))
    .uWritefln("%(%s\n%)");
}



Output (also note that 8 and 6 are printed with the array syntax):

[8, 6]
LL
ii
nn
ee

o2
n
e

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

Disadvantages:

1) A .tap() function that allows to perform some impure operation on the stream, but keep it flowing, is more general than those printing functions. Something like this:



import std.range: iota, transversal;
import std.algorithm: map, reduce, max;
import std.range: tap;

void main() {
    immutable txt = ["Line one", "Line 2"];

    txt
    .map!q{ a.length }
    .tap!q{ writeln(s) }
    .reduce!max
    .iota
    .map!(i => txt.transversal(i))
    .tap!q{ writefln("%(%s\n%)", s) };
}


In practice I've seen that in most cases I such tap only to print.



2) functions like that uWritefln() accept only the format string beside the stream to print. So you can't print the stream plus some other data. In theory this can be solved with printing functions like this:


auto uWrite(TArgs...)(TArgs items) {
    write(items);
    return items[0];
}

auto uWriteln(TArgs...)(TArgs items) {
    writeln(items);
    return items[0];
}

auto uWritef(TArgs...)(TArgs items) if (isSomeString!(TArgs[$ - 1])) {
    writef(items[$ - 1], items[0 .. $ - 1]);
    return items[0];
}

auto uWritefln(TArgs...)(TArgs items) if (isSomeString!(TArgs[$ - 1])) {
    writefln(items[$ - 1], items[0 .. $ - 1]);
    return items[0];
}


But their order of printing is not intuitive, so I suggest to not use them.


See also the discussion thread: http://forum.dlang.org/thread/vkuzklqsqqoawkbpoxpg@forum.dlang.org

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Jacob Carlborg <doob@me.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |doob@me.com


--- Comment #1 from Jacob Carlborg <doob@me.com> 2013-04-05 08:22:48 PDT ---
I like that "tap" function in Ruby and use it for non-range code as well.

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--- Comment #2 from bearophile_hugs@eml.cc 2013-04-05 11:02:48 PDT ---
(In reply to comment #1)
> I like that "tap" function in Ruby and use it for non-range code as well.

To use tap I have used a syntax like:

    .tap!q{ writeln(s) }
    .tap!q{ writefln("%(%s\n%)", s) };

But I don't know if such syntax is good and possible in D.

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--- Comment #3 from Jacob Carlborg <doob@me.com> 2013-04-05 11:46:00 PDT ---
> To use tap I have used a syntax like:
> 
>     .tap!q{ writeln(s) }
>     .tap!q{ writefln("%(%s\n%)", s) };
> 
> But I don't know if such syntax is good and possible in D.

Sure, but I prefer the lambda syntax:

.tap!(s => writeln(s))

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--- Comment #4 from bearophile_hugs@eml.cc 2013-04-05 11:52:25 PDT ---
(In reply to comment #3)
> Sure, but I prefer the lambda syntax:
> 
> .tap!(s => writeln(s))

A lambda like s => writeln(s) returns void, it's kind of the opposite of the
lambda calculus :-)

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--- Comment #5 from Jacob Carlborg <doob@me.com> 2013-04-05 12:00:05 PDT ---
(In reply to comment #4)
> A lambda like s => writeln(s) returns void, it's kind of the opposite of the
> lambda calculus :-)

Right, forgot that.

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--- Comment #6 from bearophile_hugs@eml.cc 2013-04-26 15:28:37 PDT ---
A suggestion in D.learn shows me that sometimes std.functional.binaryReverseArgs is an acceptable solution for the terminal writefln (but it can't be used for intermediate printing):


import std.stdio: writeln, writefln;
import std.range: iota, transversal;
import std.algorithm: map, reduce, max;
import std.functional: binaryReverseArgs;

void main() {
    immutable txt = ["Line one", "Line 2"];

    txt
    .map!q{ a.length }
    .map!((s){ s.writeln; return s; })
    .reduce!max
    .iota
    .map!(i => txt.transversal(i))
    .binaryReverseArgs!writefln("%(%s\n%)");
}

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