Bug 7666 – A function to reverse the items of a tuple

In some situations I'd like a function to reverse a tuple: reversed(tuple(a,b)) == reversed(tuple(b,a)) reversed(tuple(a,b,c)) == reversed(tuple(c,b,a)) reversed(tuple(a,b,c,d)) == reversed(tuple(d,c,b,a)) Once built-in associative arrays have a byPair() method, that returns a 2-tuple, a reversed(tuple(a,b)) is useful when you need the pairs in reversed (value-key) order.

First thing I could come up with, but it requires editing the Tuple!() struct. A public alias is added in Tuple struct: alias Reverse!fieldSpecs revFieldSpecs; And the internal FieldSpec struct is moved outside and made public. Then the following is possible: auto reversed(T)(T tup) if (isTuple!T) { static string getMixin(T)() { string[] result; foreach (i; 0 .. T.Types.length) { result ~= xformat("tup.field[%d]", i); } return result.retro.join(", "); } static string getSpecs(T)() { string[] result; foreach (spec; T.revFieldSpecs) { result ~= spec.Type.stringof; result ~= xformat(`"%s"`, spec.name); } return result.join(", "); } enum str = xformat("return Tuple!(%s)(%s);", getSpecs!(T)(), getMixin!(T)()); mixin(str); } void main() { alias Tuple!(int, "index", string, "value") Entry; Entry e; e.index = 4; e.value = "Hello"; auto rev = reversed(e); assert(e.index == 4); assert(e.value == "Hello"); writeln(e); writeln(rev); } Prints: Tuple!(int,"index",string,"value")(4, "Hello") Tuple!(string,"value",int,"index")("Hello", 4) If there is a non-invasive way of doing this it would be welcome.

(In reply to comment #1) > ... > > If there is a non-invasive way of doing this it would be welcome. import std.typecons, std.conv, std.range, std.algorithm; auto reversed(T)(T t) if(isTuple!T){ return mixin(`tuple(`~iota(T.length).retro.map!(a=>text("t[",a,"]")).join(",")~`)`); }

(In reply to comment #2) > (In reply to comment #1) > > ... > > > > If there is a non-invasive way of doing this it would be welcome. > With field names: import std.typecons, std.conv, std.range, std.algorithm; auto reversed(T)(T t) if(isTuple!T){ static if(is(T X:Tuple!A,A...)) alias A Spec; static struct Local{ template Seq(T...){ alias T Seq; } template RevSpec(A...){ enum num=A.length>1&&!is(A[1])?2:1; static if(A.length) alias Seq!(RevSpec!(A[num..$]),A[0..num]) RevSpec; else alias A RevSpec; } } return mixin(`Tuple!(Local.RevSpec!Spec)(`~ iota(T.length).retro.map!(a=>text("t[",a,"]")).join(",")~ `)`); }

(In reply to comment #3) > With field names Interesting, the last time I saw this syntax: static if(is(T X : Tuple!A, A...)) alias A Spec; it was in the templates book (https://github.com/PhilippeSigaud/D-templates-tutorial), where Philippe mentions: <<beg quote For me, the main limitation is that template tuple parameters are not ac- cepted. Too bad. See, imagine you use std.typecons.Tuple a lot. At one point, you need a template to test if something is a Tuple!(T...) for some T which can be 0 or more types. Though luck, is is a bit of a letdown there, as you cannot do: template isTuple(T) { static if (is(T tup == Tuple!(InnerTypes), InnerTypes...)) { enum isTuple = true; } } end quote>> Maybe this was a compiler limitation in earlier versions.

(In reply to comment #4) > ... > Maybe this was a compiler limitation in earlier versions. Yup, Kenji Hara fixed that recently.

Got something better: auto reversed(T)(T t) if (isTuple!T) { static if (is(T : Tuple!A, A...)) alias RevTypes = Reverse!A; Tuple!RevTypes result; auto tup = t.tupleof; result.tupleof = Reverse!tup; return result; } void main() { auto tup = tuple(1, "2"); assert(tup.reversed == tuple("2", 1)); } I'll make a pull shortly. P.S. this is a compile-error: Reverse!(t.tupleof) Might be worth filing this as a bug.

Commits pushed to master at https://github.com/D-Programming-Language/phobos https://github.com/D-Programming-Language/phobos/commit/cc7c5c5ed2b2e345a29f5e8a663db4556eb8819e Fixes Issue 7666 - Implement function to reverse fields of a tuple. https://github.com/D-Programming-Language/phobos/commit/5c2890f23eddb8ac72ed57e44e1c54b6f43593e1 Merge pull request #1244 from AndrejMitrovic/Fix7666 Issue 7666 - Implement function to reverse fields of a tuple.

So sorry about this, I completely forgot about the string specs feature of Tuples. I guess I was a little sleepy. Will re-do a pull tomorrow.

(In reply to comment #9) > So sorry about this, I completely forgot about the string specs feature of > Tuples. I guess I was a little sleepy. Will re-do a pull tomorrow. Also as reported by Walter it seems we get frame pointer errors when trying to use it from within a unittest.

Commits pushed to master at https://github.com/D-Programming-Language/phobos https://github.com/D-Programming-Language/phobos/commit/f9994612b8bc10133425eb144731657b0806618f Fixes Issue 7666 - Implement reverse property for Tuple. https://github.com/D-Programming-Language/phobos/commit/8a3962b778eafc98c563cfa58a659dd41a2e2338 Merge pull request #1248 from AndrejMitrovic/Fix7666_2 Issue 7666 - Implement reverse property for Tuple.

The presence of this function solves the problem of choosing what's the right API for the Phobos-defined array.reverse: for API uniformity is should be like this one, so array.reverse should reverse the items in-place (unlike this Tuple.reverse) and return the reversed items, to be used in UFCS chains.