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digitalmars.D - parameter type tuple or alias for std.traits templates ?

reply Guillaume Chatelet <chatelet.guillaume gmail.com> writes:
While reading the source code of std.traits I came across a bunch of
variadic templates enforcing the length of their parameter type tuple to
be of 1.

Is there any semantic difference between

 template ReturnType(func...) if (func.length == 1 && isCallable!func)




 template ReturnType(alias func) if (isCallable!func)



If not, is there any reason why this syntax is used ?

Guillaume
--
The list of templates from std.traits (
https://github.com/D-Programming-Language/phobos/blob/master/std/traits.d )
matching this pattern :

template ReturnType(func...)
 if (func.length == 1 && isCallable!func)

template ParameterTypeTuple(func...)
 if (func.length == 1 && isCallable!func)

template ParameterStorageClassTuple(func...)
 if (func.length == 1 && isCallable!func)

template functionAttributes(func...)
 if (func.length == 1 && isCallable!func)

template functionLinkage(func...)
 if (func.length == 1 && isCallable!func)

template variadicFunctionStyle(func...)
 if (func.length == 1 && isCallable!func)

template FunctionTypeOf(func...)
 if (func.length == 1 && isCallable!func)

template isFunctionPointer(T...)
 if (T.length == 1)

template isDelegate(T...)
 if(T.length == 1)

template isSomeFunction(T...)
 if (T.length == 1)

template isCallable(T...)
 if (T.length == 1)

template isAbstractFunction(method...)
 if (method.length == 1)

template mangledName(sth...)
 if (sth.length == 1)
Jun 22 2012
parent reply Timon Gehr <timon.gehr gmx.ch> writes:
On 06/22/2012 11:50 PM, Guillaume Chatelet wrote:

 While reading the source code of std.traits I came across a bunch of
 variadic templates enforcing the length of their parameter type tuple to
 be of 1.

 Is there any semantic difference between

 template ReturnType(func...) if (func.length == 1&&  isCallable!func)




 template ReturnType(alias func) if (isCallable!func)






Yes there is, but I have argued that this is a bug in the past. With 
DMD, the (func...) accepts built-in types, while the (alias func) does
not.



 If not, is there any reason why this syntax is used ?

 Guillaume
 --
Jun 22 2012
next sibling parent Guillaume Chatelet <chatelet.guillaume gmail.com> writes:
On 06/23/12 01:05, Timon Gehr wrote:

 
 Yes there is, but I have argued that this is a bug in the past. With
 DMD, the (func...) accepts built-in types, while the (alias func) does
 not.



Ok got it, the following won't compile I agree.

template isInt(alias T) {
	enum bool isInt = is(T==int);
}

void main(){
	static assert(isInt!int);
}

So I understand the following functions have to go the variadic way
because you want an answer at compile time and not a compiler error
template isFunctionPointer(T...)
template isDelegate(T...)
template isSomeFunction(T...)
template isCallable(T...)
template isAbstractFunction(method...)
template mangledName(sth...)

Nevertheless the following templates are all requiring 'func.length == 1
&& isCallable!func'

template ReturnType(func...)
template ParameterTypeTuple(func...)
template ParameterStorageClassTuple(func...)
template functionAttributes(func...)
template functionLinkage(func...)
template variadicFunctionStyle(func...)
template FunctionTypeOf(func...)

So they won't instantiate if you pass a built-in type anyway. Using an
'alias' instead of a 'func...' would make it clear that :
* you can't pass a built-in type in
* the template is requiring exactly one argument

As a user, I find it odd that ReturnType takes a TypeTuple rather than
an alias. 'func...' basically means 'I will accept a tuple of functions'
which is then restricted in the if clause to the case where func.length==1.

--
Guillaume
Jun 23 2012
prev sibling next sibling parent reply Jacob Carlborg <doob me.com> writes:
On 2012-06-23 01:05, Timon Gehr wrote:


 Yes there is, but I have argued that this is a bug in the past. With
 DMD, the (func...) accepts built-in types, while the (alias func) does
 not.



Would it be possible to overload? Something like this:

template Foo (T) {}

template Foo (alias a) {}

-- 
/Jacob Carlborg
Jun 23 2012
parent Timon Gehr <timon.gehr gmx.ch> writes:
On 06/23/2012 12:52 PM, Jacob Carlborg wrote:

 On 2012-06-23 01:05, Timon Gehr wrote:


 Yes there is, but I have argued that this is a bug in the past. With
 DMD, the (func...) accepts built-in types, while the (alias func) does
 not.



 Would it be possible to overload? Something like this:

 template Foo (T) {}

 template Foo (alias a) {}



alias should just accept built in types.
Jun 23 2012
prev sibling next sibling parent Guillaume Chatelet <chatelet.guillaume gmail.com> writes:
I'm reposting this because I'm not sure it made it to the NG ( it
doesn't show up in the web interface )

On 06/23/12 01:05, Timon Gehr wrote:

 Yes there is, but I have argued that this is a bug in the past. With
 DMD, the (func...) accepts built-in types, while the (alias func) does
 not.



Ok got it, the following won't compile I agree.

template isInt(alias T) {
	enum bool isInt = is(T==int);
}

void main(){
	static assert(isInt!int);
}

So I understand the following functions have to go the variadic way
because you want an answer at compile time and not a compiler error
template isFunctionPointer(T...)
template isDelegate(T...)
template isSomeFunction(T...)
template isCallable(T...)
template isAbstractFunction(method...)
template mangledName(sth...)

Nevertheless the following templates are all requiring 'func.length == 1
&& isCallable!func'

template ReturnType(func...)
template ParameterTypeTuple(func...)
template ParameterStorageClassTuple(func...)
template functionAttributes(func...)
template functionLinkage(func...)
template variadicFunctionStyle(func...)
template FunctionTypeOf(func...)

So they won't instantiate if you pass a built-in type anyway. Using an
'alias' instead of a 'func...' would make it clear that :
* you can't pass a built-in type in
* the template is requiring exactly one argument

As a user, I find it odd that ReturnType takes a TypeTuple rather than
an alias. 'func...' basically means 'I will accept a tuple of functions'
which is then restricted in the if clause to the case where func.length==1.

--
Guillaume
Jun 23 2012
prev sibling parent Philippe Sigaud <philippe.sigaud gmail.com> writes:
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Le 23 juin 2012 12:53, "Jacob Carlborg" <doob me.com> a =C3=A9crit :

 On 2012-06-23 01:05, Timon Gehr wrote:


 Yes there is, but I have argued that this is a bug in the past. With
 DMD, the (func...) accepts built-in types, while the (alias func) does
 not.




 Would it be possible to overload? Something like this:

 template Foo (T) {}

 template Foo (alias a) {}



I think it is. IIRC, I did something like this.

But then the trouble is with user-defined types: they are both a type and a
symbol, so they may fire both templates.

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<p><br>
Le 23 juin 2012 12:53, &quot;Jacob Carlborg&quot; &lt;<a href=3D"mailto:doo=
b me.com">doob me.com</a>&gt; a =C3=A9crit=C2=A0:<br>
&gt;<br>
&gt; On 2012-06-23 01:05, Timon Gehr wrote:<br>
&gt;<br>
&gt;&gt; Yes there is, but I have argued that this is a bug in the past. Wi=
th<br>
&gt;&gt; DMD, the (func...) accepts built-in types, while the (alias func) =
does<br>
&gt;&gt; not.<br>
&gt;<br>
&gt;<br>
&gt; Would it be possible to overload? Something like this:<br>
&gt;<br>
&gt; template Foo (T) {}<br>
&gt;<br>
&gt; template Foo (alias a) {}</p>
<p>I think it is. IIRC, I did something like this.</p>
<p>But then the trouble is with user-defined types: they are both a type an=
d a symbol, so they may fire both templates.<br>
</p>

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Jun 23 2012