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Issue 6521 - writeln(const(tuple)) doesn't show the field values
Summary: writeln(const(tuple)) doesn't show the field values
Status: RESOLVED FIXED
Alias: None
Product: D
Classification: Unclassified
Component: phobos (show other issues)
Version: D2
Hardware: x86 Windows
: P2 normal
Assignee: No Owner
URL:
Keywords:
Depends on:
Blocks:
 
Reported: 2011-08-17 07:49 UTC by bearophile_hugs
Modified: 2011-09-09 08:03 UTC (History)
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Description bearophile_hugs 2011-08-17 07:49:07 UTC 
import std.stdio: writeln;
import std.typecons: tuple;
void main() {
    auto r1 = tuple(1);
    writeln(r1);
    const r2 = tuple(1);
    writeln(r2);
}


2.055beta prints:

Tuple!(int)(1)
const(Tuple!(int))


But I'd like to see the values of the fields (1) in the second case too.
Comment 1 Kenji Hara 2011-08-17 18:15:36 UTC 
Expected prints:

Tuple!(int)(1)
const(Tuple!(int))(1)

There are two ways to fix the problem.

1. Change std.typecons.Tuple!T.toString like follows:

diff --git a/std/typecons.d b/std/typecons.d
index b279abf..b80d6f9 100644
--- a/std/typecons.d
+++ b/std/typecons.d
@@ -491,9 +491,9 @@ assert(s[0] == "abc" && s[1] == 4.5);
 /**
    Converts to string.
  */
-    string toString()
+    const string toString(this T)()
     {
-        enum header = typeof(this).stringof ~ "(",
+        enum header = T.stringof ~ "(",
              footer = ")",
              separator = ", ";
         Appender!string app;

But this is very hackish to me.

2. Apply my pull request
https://github.com/D-Programming-Language/phobos/pull/126
This patch changes formattedWrite behavior: if a struct type does not have toString method, formattedWrite formats it like POD (e.g. "TypeName(field, ...)").

I think that Tuple is just a POD type, and expected formatting also looks like the POD.
So, IMHO, std.typecons.Tuple!T.toString is *just a workaround*.
If formattedWrite works correctly, we will no longer need Tuple!T.toString.
Comment 2 bearophile_hugs 2011-08-17 19:49:15 UTC 
(In reply to comment #1)

> There are two ways to fix the problem.
> 
> 1. Change std.typecons.Tuple!T.toString like follows:

> 2. Apply my pull request

> So, IMHO, std.typecons.Tuple!T.toString is *just a workaround*.
> If formattedWrite works correctly, we will no longer need Tuple!T.toString.

I agree that the point 2 is better.

Your Phobos pull 126 looks exceptionally good, it seems to solve most problems I have with D printing since years. Thank you very much for it.

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

But I have one more note. Producing a good print is a matter of balances and tradeoffs. When I print many tuples that contain several fields (with field names too), the output is currently very noisy and very long. I see some alternative solutions to this problem:

1) Keep things as in you pull 126. So every tuple gets printed like:
Tuple!(int, "x", double, "dx")(1, 1.5)


2) Use a shorter textual representation for tuples, that shows no types and no field names. Example:
tuple(1, 1.5)
Or even just:
(1, 1.5)

3) Find some intermediate way. Example: print single tuples fully:
Tuple!(int, "x", double, "dx")(1, 1.5)
but print arrays of tuples in a more compact way:
[tuple(1, 1.5), tuple(2, 2.5)]