Hello.

Today I was looking at the grammar for template value parameters and realized I didn't know what the conditional expression is for in

TemplateValueParameter => BasicType Declarator : ConditionalExpression

So what does it do?

(I assume = ConditionalExpression is the default value for the parameter)

Hello Ellery,

> Hello.
> 
> Today I was looking at the grammar for template value parameters and
> realized I didn't know what the conditional expression is for in
> 
> TemplateValueParameter => BasicType Declarator : ConditionalExpression
> 
> So what does it do?
> 
> (I assume = ConditionalExpression is the default value for the
> parameter)
> 

That is correct. it would seem that expression should be used, but that includes an assignment expression so the next things down is used.

http://www.digitalmars.com/d/1.0/expression.html#AssignExpression

BCS wrote:
> Hello Ellery,
> 
>> Hello.
>>
>> Today I was looking at the grammar for template value parameters and realized I didn't know what the conditional expression is for in
>>
>> TemplateValueParameter => BasicType Declarator : ConditionalExpression
>>
>> So what does it do?
>>
>> (I assume = ConditionalExpression is the default value for the
>> parameter)
>>
> 
> That is correct. it would seem that expression should be used, but that includes an assignment expression so the next things down is used.
> 
> http://www.digitalmars.com/d/1.0/expression.html#AssignExpression
> 
> 

Okay, let me rephrase that:

When you have a <something> preceded by a colon in a template value parameter, what are its semantics? Is it the same as a default parameter value? Is it some sort of constraining expression? Is it a vestige?

Ellery Newcomer wrote:
> Okay, let me rephrase that:
> 
> When you have a <something> preceded by a colon in a template value
> parameter, what are its semantics? Is it the same as a default parameter
> value? Is it some sort of constraining expression? Is it a vestige?

void foo(T : int)(T value)
{
	// value is implicitly convertible to int
}

It can do some pattern matching:

void foo(T : V[U], V, U)(T dictionary)
{
	// I have an associative array
}

In D2, it's usually going to be easier to use constraints:

template foo(T) if (isAssociativeArray!T) {}

Christopher Wright wrote:
> Ellery Newcomer wrote:
>> Okay, let me rephrase that:
>>
>> When you have a <something> preceded by a colon in a template value parameter, what are its semantics? Is it the same as a default parameter value? Is it some sort of constraining expression? Is it a vestige?
> 
> void foo(T : int)(T value)
> {
>     // value is implicitly convertible to int
> }
> 
> It can do some pattern matching:
> 
> void foo(T : V[U], V, U)(T dictionary)
> {
>     // I have an associative array
> }
> 
> In D2, it's usually going to be easier to use constraints:
> 
> template foo(T) if (isAssociativeArray!T) {}

Great, it does something specified for a type parameter.

Now what does it do for a VALUE parameter?

On Fri, Sep 25, 2009 at 6:48 PM, Ellery Newcomer <ellery-newcomer@utulsa.edu> wrote:
> Christopher Wright wrote:
>> Ellery Newcomer wrote:
>>> Okay, let me rephrase that:
>>>
>>> When you have a <something> preceded by a colon in a template value parameter, what are its semantics? Is it the same as a default parameter value? Is it some sort of constraining expression? Is it a vestige?
>>
>> void foo(T : int)(T value)
>> {
>>     // value is implicitly convertible to int
>> }
>>
>> It can do some pattern matching:
>>
>> void foo(T : V[U], V, U)(T dictionary)
>> {
>>     // I have an associative array
>> }
>>
>> In D2, it's usually going to be easier to use constraints:
>>
>> template foo(T) if (isAssociativeArray!T) {}
>
> Great, it does something specified for a type parameter.
>
> Now what does it do for a VALUE parameter?

Specialization for particular values. It's not really that useful in the face of static if.

template fib(int n : 0) { const fib = 1; }
template fib(int n : 1) { const fib = 1; }
template fib(int n) { const fib = fib!(n - 2) + fib!(n - 1); }

Hello Ellery,

> BCS wrote:
> 
>> Hello Ellery,
>> 
>>> Hello.
>>> 
>>> Today I was looking at the grammar for template value parameters and
>>> realized I didn't know what the conditional expression is for in
>>> 
>>> TemplateValueParameter => BasicType Declarator :
>>> ConditionalExpression
>>> 
>>> So what does it do?
>>> 
>>> (I assume = ConditionalExpression is the default value for the
>>> parameter)
>>> 
>> That is correct. it would seem that expression should be used, but
>> that includes an assignment expression so the next things down is
>> used.
>> 
>> http://www.digitalmars.com/d/1.0/expression.html#AssignExpression
>> 
> Okay, let me rephrase that:
> 
> When you have a <something> preceded by a colon in a template value
> parameter, what are its semantics? Is it the same as a default
> parameter value? Is it some sort of constraining expression? Is it a
> vestige?
>

You should check me on this but I think it's explicit instantiation: "This is the special case for when given some value".