• Chris Williams (27/27) Feb 12 2015 I have a template function that gets values out of a tree of
• ketmar (3/4) Feb 12 2015 there is at least `std.traits.isImplicitlyConvertible`:
• Jonathan M Davis via Digitalmars-d-learn (13/40) Feb 12 2015 There are traits in std.traits for testing stuff like

"Chris Williams" <yoreanon-chrisw yahoo.co.jp> writes:

I have a template function that gets values out of a tree of 
variant types. My goal is to be able to write code like;

node.get!string("path", "to", "leaf");

Inside get(), I would like to use std.conv to dynamically convert 
(where able) to the target type (T) or, if that is not possible, 
to return T.init.

If I wanted to force the user to request the correct type as is 
stored in the structure, I could write code like:

switch (leafType) {
case STRING:
    static if (is(T : string)) {
       return leftValue;
    }
    else {
       return T.init
    }
break;
...

But since I want to allow all possiblities that std.conv 
supports, I want something like:

static if (isConvertible!(T, string)) {
    return leftValue.to!T;
}
else {
    return T.init;
}

Is there something like isConvertible() in the library somewhere?

ketmar <ketmar ketmar.no-ip.org> writes:

On Thu, 12 Feb 2015 19:09:45 +0000, Chris Williams wrote:

 Is there something like isConvertible() in the library somewhere?

there is at least `std.traits.isImplicitlyConvertible`:
http://dlang.org/phobos/std_traits.html#isImplicitlyConvertible=

Jonathan M Davis via Digitalmars-d-learn writes:

On Thursday, February 12, 2015 19:09:45 Chris Williams via Digitalmars-d-learn
wrote:
 I have a template function that gets values out of a tree of
 variant types. My goal is to be able to write code like;

 node.get!string("path", "to", "leaf");

 Inside get(), I would like to use std.conv to dynamically convert
 (where able) to the target type (T) or, if that is not possible,
 to return T.init.

 If I wanted to force the user to request the correct type as is
 stored in the structure, I could write code like:

 switch (leafType) {
 case STRING:
     static if (is(T : string)) {
        return leftValue;
     }
     else {
        return T.init
     }
 break;
 ...

 But since I want to allow all possiblities that std.conv
 supports, I want something like:

 static if (isConvertible!(T, string)) {
     return leftValue.to!T;
 }
 else {
     return T.init;
 }

 Is there something like isConvertible() in the library somewhere?

There are traits in std.traits for testing stuff like
isImplicitlyConvertibel, but std.conv.to is _way_ too fancy to expect any of
the traits in std.traits to tell you whether std.conv.to supports a
particular conversion (and there's every possibility that what std.conv.to
can do will increase from one release to another). By far the simplest way
(and probably the _only_ way realistically) would be to test std.conv.to
itself. Something like

is(typeof({auto t = to!T(string.init);}))

should do the trick for strings, and if you have a variable that you want to
convert from and not just strings, then you could do something like

is(typeof({auto t = to!T(myVar);}))

- Jonathan M Davis