Re: Introduction to traits (and __traits)
Am 30.08.2013 21:36, schrieb Joseph Rushton Wakeling:
Hello all,
I find myself wanting to write for the first time one of these
isSomething(T) or hasSomething(T) templates that perform compile-time
checks, so I was hoping people could give me some good general advice on
traits.
The goal here is to be able to confirm (i) type T has certain members
and (ii) they have certain properties. (This is for my graph library,
Dgraph.)
So, to start off with, I thought I'd try starting with,
template isGraph(G)
{
isGraph = __traits(hasMember, G, directed)
isBoolean!(typeof(G.directed));
}
... which I'd assumed would cover the case where G does not have a
member directed. But in fact if I pass it a struct that does not have
the entity .directed defined therein, it will return an error: no
property 'directed' for type ...
So, can someone give me a good idea of how to go about writing such a
compile-time template that checks (i) for the existence of certain
methods/functions/members and (ii) confirms their characteristics, such
as their return values or arguments?
Thanks best wishes,
-- Joe
You need to put it into a static if, otherwise the compiler will
continue semantic checks on the second part of the expression. E.g.
template isGraph(G)
{
static if(__traits(hasMember, G, directed))
enum bool isGraph = isBoolean!(typeof(G.directed));
else
enum bool isGraph = false;
}
Introduction to traits (and __traits)
Hello all,
I find myself wanting to write for the first time one of these isSomething(T) or
hasSomething(T) templates that perform compile-time checks, so I was hoping
people could give me some good general advice on traits.
The goal here is to be able to confirm (i) type T has certain members and (ii)
they have certain properties. (This is for my graph library, Dgraph.)
So, to start off with, I thought I'd try starting with,
template isGraph(G)
{
isGraph = __traits(hasMember, G, directed)
isBoolean!(typeof(G.directed));
}
... which I'd assumed would cover the case where G does not have a member
directed. But in fact if I pass it a struct that does not have the entity
.directed defined therein, it will return an error: no property 'directed' for
type ...
So, can someone give me a good idea of how to go about writing such a
compile-time template that checks (i) for the existence of certain
methods/functions/members and (ii) confirms their characteristics, such as their
return values or arguments?
Thanks best wishes,
-- Joe
Re: Introduction to traits (and __traits)
On 30/08/13 21:40, Benjamin Thaut wrote:
You need to put it into a static if, otherwise the compiler will continue
semantic checks on the second part of the expression. E.g.
template isGraph(G)
{
static if(__traits(hasMember, G, directed))
enum bool isGraph = isBoolean!(typeof(G.directed));
else
enum bool isGraph = false;
}
Ahh, right, thanks. :-)
Is there a recommended way for handling the case where there are many such
members -- say about 10 or more? The static if's could become very highly
nested with this approach. I suppose I could go through like this:
static if(!__traits(hasMember, G, one))
enum bool isGraph = false;
else static if(!__traits(hasMember, G, two))
enum bool isGraph = false;
else static if ...
...
else
{
// Now I know all the members exist and I can
// start checking out their properties ...
}
Re: Introduction to traits (and __traits)
On 30/08/13 23:06, Ali Çehreli wrote:
How about allSatisfy:
http://dlang.org/phobos/std_typetuple.html#.allSatisfy
I'll have a look at that, thanks :-)
Here's what I came up with, for now. It should probably be extended with
further tests on the characteristics of the various member functions but it
seems to be sufficient for now:
template isGraph(G)
{
static if (!__traits(hasMember, G, directed) ||
!__traits(hasMember, G, edge) ||
!__traits(hasMember, G, edgeCount) ||
!__traits(hasMember, G, vertexCount) ||
!__traits(hasMember, G, isEdge) ||
!__traits(hasMember, G, edgeID) ||
!__traits(hasMember, G, addEdge) ||
!__traits(hasMember, G, degreeIn) ||
!__traits(hasMember, G, degreeOut) ||
!__traits(hasMember, G, incidentEdgesIn) ||
!__traits(hasMember, G, incidentEdgesOut) ||
!__traits(hasMember, G, neighboursIn) ||
!__traits(hasMember, G, neighboursOut))
{
enum bool isGraph = false;
}
else static if (!isBoolean!(typeof(G.directed)))
{
enum bool isGraph = false;
}
else static if (G.directed (__traits(hasMember, G, degree) ||
__traits(hasMember, G, incidentEdges) ||
__traits(hasMember, G, neighbours)))
{
enum bool isGraph = false;
}
else static if (!G.directed (!__traits(hasMember, G, degree) ||
!__traits(hasMember, G, incidentEdges) ||
!__traits(hasMember, G, neighbours)))
{
enum bool isGraph = false;
}
else
{
enum bool isGraph = true;
}
}
Re: Introduction to traits (and __traits)
On 08/30/2013 01:57 PM, Joseph Rushton Wakeling wrote:
On 30/08/13 21:40, Benjamin Thaut wrote:
You need to put it into a static if, otherwise the compiler will continue
semantic checks on the second part of the expression. E.g.
template isGraph(G)
{
static if(__traits(hasMember, G, directed))
enum bool isGraph = isBoolean!(typeof(G.directed));
else
enum bool isGraph = false;
}
Ahh, right, thanks. :-)
Is there a recommended way for handling the case where there are many
such members -- say about 10 or more? The static if's could become very
highly nested with this approach. I suppose I could go through like this:
static if(!__traits(hasMember, G, one))
enum bool isGraph = false;
else static if(!__traits(hasMember, G, two))
enum bool isGraph = false;
else static if ...
...
else
{
// Now I know all the members exist and I can
// start checking out their properties ...
}
How about allSatisfy:
http://dlang.org/phobos/std_typetuple.html#.allSatisfy
Ali
Re: Introduction to traits (and __traits)
On Fri, Aug 30, 2013 at 11:51:37PM +0200, bearophile wrote:
Joseph Rushton Wakeling:
static if (!__traits(hasMember, G, directed) ||
!__traits(hasMember, G, edge) ||
!__traits(hasMember, G, edgeCount) ||
!__traits(hasMember, G, vertexCount) ||
!__traits(hasMember, G, isEdge) ||
!__traits(hasMember, G, edgeID) ||
!__traits(hasMember, G, addEdge) ||
!__traits(hasMember, G, degreeIn) ||
!__traits(hasMember, G, degreeOut) ||
!__traits(hasMember, G, incidentEdgesIn) ||
!__traits(hasMember, G, incidentEdgesOut) ||
!__traits(hasMember, G, neighboursIn) ||
!__traits(hasMember, G, neighboursOut))
Perhaps can shorten that code writing a hasMembers helper (and I
suggest to keep those names sorted):
[...]
Here's a first stab at a possible implementation:
/* Warning: untested code */
import std.typetuple : allSatisfy;
template isString(T) {
// There may already be something in Phobos that does
// this, but I'm too lazy to look.
enum isString = is(T == string);
}
template hasMembers(alias T, Members...)
if (allSatisfy!isString(Members))
{
// Template recursion + exprTuple slicing FTW :)
enum hasMembers = __traits(hasMember, T, Members[0])
hasMembers!(T, Members[1..$]);
}
void myFunc(G)(G graph)
{
static if (hasMembers!(G, directed, edge, /* ... */))
{
...
}
}
T
--
Computers shouldn't beep through the keyhole.
Re: Introduction to traits (and __traits)
Joseph Rushton Wakeling:
static if (!__traits(hasMember, G, directed) ||
!__traits(hasMember, G, edge) ||
!__traits(hasMember, G, edgeCount) ||
!__traits(hasMember, G, vertexCount) ||
!__traits(hasMember, G, isEdge) ||
!__traits(hasMember, G, edgeID) ||
!__traits(hasMember, G, addEdge) ||
!__traits(hasMember, G, degreeIn) ||
!__traits(hasMember, G, degreeOut) ||
!__traits(hasMember, G, incidentEdgesIn) ||
!__traits(hasMember, G, incidentEdgesOut) ||
!__traits(hasMember, G, neighboursIn) ||
!__traits(hasMember, G, neighboursOut))
Perhaps can shorten that code writing a hasMembers helper (and I
suggest to keep those names sorted):
static if (hasMembers!(G, addEdge
degreeIn
...
vertexCount.split) {
Bye,
bearophile
Re: Introduction to traits (and __traits)
On 30/08/13 23:51, bearophile wrote: Perhaps can shorten that code writing a hasMembers helper (and I suggest to keep those names sorted) Nice thought, I might look into that at some point. :-)
