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digitalmars.D.learn - how do I get the ith field of a std.typecons.Tuple ?

reply Timothee Cour <thelastmammoth gmail.com> writes:
A)
how do I get the ith field of a std.typecons.Tuple ?
ideally, it should be as simple as:

auto t=Tuple!(int,"name",double,"name2")(1);
static assert(t.fields[0] == "name");

It seems the necessary items are private, so how do I get the ith field of
a std.typecons.Tuple ?
I really don't want to parse T.stringof, which could require a full parser
(eg: with Tuple!(A!"bar","name") )

B)
Related question:
Why isn't slicing and indexing allowed for the Tuple type?
eg:
alias T=typeof(t);
static assert(is(T[0] == int));
static assert(is(T[0..1] == Tuple!(int,"name"));

C)
Same with appending:
static assert(is(T[0..1]~T[1..2] == T));

D)
I'm trying to make an append operator but because of problem (A) above I
can't (for named Tuples).
Aug 18 2013
next sibling parent reply "John Colvin" <john.loughran.colvin gmail.com> writes:
On Sunday, 18 August 2013 at 08:46:17 UTC, Timothee Cour wrote:

 A)
 how do I get the ith field of a std.typecons.Tuple ?
 ideally, it should be as simple as:

 auto t=Tuple!(int,"name",double,"name2")(1);
 static assert(t.fields[0] == "name");


field is the old name for expand, retained for compatibility, 
it's not recommended. It gives you direct access to the tuple 
inside the Tuple struct, but it's for getting the variable 
values, not their names.

If you want to get the *names* you've chosen for the tuple 
fields, you'll have to use traits of some sort I think.
Aug 18 2013
next sibling parent Timothee Cour <thelastmammoth gmail.com> writes:
On Sun, Aug 18, 2013 at 2:15 AM, John Colvin <john.loughran.colvin gmail.com

 wrote:



 On Sunday, 18 August 2013 at 08:46:17 UTC, Timothee Cour wrote:


 A)
 how do I get the ith field of a std.typecons.Tuple ?
 ideally, it should be as simple as:

 auto t=Tuple!(int,"name",double,"**name2")(1);
 static assert(t.fields[0] == "name");


 field is the old name for expand, retained for compatibility, it's not
 recommended. It gives you direct access to the tuple inside the Tuple
 struct, but it's for getting the variable values, not their names.


I didn't mean Tuple.field (as in Tuple.expand), I really meant the name of
the corresponding entry, as shown in my example.

If you want to get the *names* you've chosen for the tuple fields, you'll

 have to use traits of some sort I think.


I don't see how that would work, however I've figured out how to do it:

That's a bit of a hack, but should work. Should it be included in phobos,
or, better, shall we fix Tuple with some of the recommendations i gave
above?
----
import std.typecons;
auto tupleField(T,size_t i)()if(isTuple!T && i<T.length){
  enum foo0=typeof(T.init.slice!(i,i+1)).stringof;
  static assert(foo0[$-2..$]==`")`);//otherwise not a tuple with fields
  enum foo=typeof(T.init.slice!(i,i+1)).stringof[0..$-2];
  size_t j=foo.length;
  while(true){
    char fj=foo[--j];
    if(fj=='"')
      return foo[j+1..$];
  }
}
unittest{
  import std.typecons;
  auto t=Tuple!(int,"foo",double,"bar")(2,3.4);
  alias T=typeof(t);
  static assert(tupleField!(T,0)=="foo");
  static assert(tupleField!(T,1)=="bar");
}
----
Aug 18 2013
prev sibling parent Timothee Cour <thelastmammoth gmail.com> writes:
and this:
auto tupleFields(T)()if(isTuple!T){
  string[T.length]ret;
  foreach(i;Iota!(T.length))
    ret[i]=tupleField!(T,i);
  return ret;
}
unittest{
  import std.typecons;
  auto t=Tuple!(int,"foo",double,"bar")(2,3.4);
  alias T=typeof(t);
  static assert(tupleFields!T==["foo","bar"]);
}



On Sun, Aug 18, 2013 at 2:26 AM, Timothee Cour <thelastmammoth gmail.com>wrote:


 On Sun, Aug 18, 2013 at 2:15 AM, John Colvin <
 john.loughran.colvin gmail.com> wrote:


 On Sunday, 18 August 2013 at 08:46:17 UTC, Timothee Cour wrote:


 A)
 how do I get the ith field of a std.typecons.Tuple ?
 ideally, it should be as simple as:

 auto t=Tuple!(int,"name",double,"**name2")(1);
 static assert(t.fields[0] == "name");


 field is the old name for expand, retained for compatibility, it's not
 recommended. It gives you direct access to the tuple inside the Tuple
 struct, but it's for getting the variable values, not their names.


 I didn't mean Tuple.field (as in Tuple.expand), I really meant the name of
 the corresponding entry, as shown in my example.

 If you want to get the *names* you've chosen for the tuple fields, you'll

 have to use traits of some sort I think.


 I don't see how that would work, however I've figured out how to do it:

 That's a bit of a hack, but should work. Should it be included in phobos,
 or, better, shall we fix Tuple with some of the recommendations i gave
 above?
 ----
 import std.typecons;
 auto tupleField(T,size_t i)()if(isTuple!T && i<T.length){
   enum foo0=typeof(T.init.slice!(i,i+1)).stringof;
   static assert(foo0[$-2..$]==`")`);//otherwise not a tuple with fields
   enum foo=typeof(T.init.slice!(i,i+1)).stringof[0..$-2];
   size_t j=foo.length;
   while(true){
     char fj=foo[--j];
     if(fj=='"')
       return foo[j+1..$];
   }
 }
 unittest{
   import std.typecons;
   auto t=Tuple!(int,"foo",double,"bar")(2,3.4);
   alias T=typeof(t);
   static assert(tupleField!(T,0)=="foo");
   static assert(tupleField!(T,1)=="bar");
 }
 ----
Aug 18 2013
prev sibling parent reply "Dicebot" <public dicebot.lv> writes:
Looking at Tuple implementation, this information gets lost at 
template instatiation time. I think it is worth a pull request to 
store properly ordered tuple of field aliases in Tuple type.
Aug 18 2013
parent reply "John Colvin" <john.loughran.colvin gmail.com> writes:
On Sunday, 18 August 2013 at 18:56:02 UTC, Dicebot wrote:

 Looking at Tuple implementation, this information gets lost at 
 template instatiation time. I think it is worth a pull request 
 to store properly ordered tuple of field aliases in Tuple type.


To get the names: 
https://github.com/D-Programming-Language/phobos/pull/1493
Aug 18 2013
parent reply "Joseph Rushton Wakeling" <joseph.wakeling webdrake.net> writes:
On Sunday, 18 August 2013 at 19:48:39 UTC, John Colvin wrote:

 On Sunday, 18 August 2013 at 18:56:02 UTC, Dicebot wrote:

 Looking at Tuple implementation, this information gets lost at 
 template instatiation time. I think it is worth a pull request 
 to store properly ordered tuple of field aliases in Tuple type.


 To get the names: 
 https://github.com/D-Programming-Language/phobos/pull/1493


Just checking, as I'm not too familiar with staticMap, but ... is 
that something that will vanish nicely at runtime?  That is, it 
won't add extra weight to the Tuple that must be passed around?
Aug 18 2013
parent "John Colvin" <john.loughran.colvin gmail.com> writes:
On Sunday, 18 August 2013 at 20:54:14 UTC, Joseph Rushton 
Wakeling wrote:

 On Sunday, 18 August 2013 at 19:48:39 UTC, John Colvin wrote:

 On Sunday, 18 August 2013 at 18:56:02 UTC, Dicebot wrote:

 Looking at Tuple implementation, this information gets lost 
 at template instatiation time. I think it is worth a pull 
 request to store properly ordered tuple of field aliases in 
 Tuple type.


 To get the names: 
 https://github.com/D-Programming-Language/phobos/pull/1493


 Just checking, as I'm not too familiar with staticMap, but ... 
 is that something that will vanish nicely at runtime?  That is, 
 it won't add extra weight to the Tuple that must be passed 
 around?


Doesn't add any any runtime weight at all, it's just an alias.
Aug 18 2013