Bug 6521 – writeln(const(tuple)) doesn't show the field values

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.

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.

(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)]

Fixed. https://github.com/D-Programming-Language/phobos/commit/4c8cbd2f29637abfadb2d3057a5e747fe8084d4d