Re: Pointer to dlang spec for this alias construct?
On Monday, 17 June 2024 at 05:05:06 UTC, Jonathan M Davis wrote: alias Unshared(T) = T; alias Unshared(T: shared U, U) = U; ... Unshared is an eponymous template. https://dlang.org/spec/template.html#implicit_template_properties And it's using a shortcut syntax. ... The second template uses a template specialization https://dlang.org/spec/template.html#parameters_specialization No wonder I couldn't find it in the spec; I was looking for an "alias" language feature. alias as used here leans on a template mechanism--specialization. It might have taken many readings of the spec to have hunted that down, so--thank you. It's quite an interesting mechanism. D is quite easy for a C programmer to get started with, but it certainly has its depths! Andy
Re: Pointer to dlang spec for this alias construct?
On Sunday, June 16, 2024 10:32:50 PM MDT Andy Valencia via Digitalmars-d-learn
wrote:
> In the alias:
>
> alias Unshared(T) = T;
> alias Unshared(T: shared U, U) = U;
>
> as used in:
>
> cast(Unshared!mytype)value
>
> turns a mytype with shared attribute into one without shared.
>
> I deduce the alias is using some sort of type matching and
> decomposition?
>
> I've read through the language spec, and didn't spot this
> mechanism. Can somebody tell me what section of the spec covers
> this very interesting mechanism?
>
> Thanks in advance,
> Andy
Unshared is an eponymous template.
https://dlang.org/spec/template.html#implicit_template_properties
And it's using a shortcut syntax.
alias Unshared(T) = T;
alias Unshared(T: shared U, U) = U;
is equivalent to
template Unshared(T)
{
alias Unshared = T;
}
template Unshared(T : shared U, U)
{
alias Unshared = U;
}
The alias is simply a normal alias that's the result of evaluating the
template.
https://dlang.org/spec/declaration.html#alias
The second template uses a template specialization
https://dlang.org/spec/template.html#parameters_specialization
that matches based on the fact that the given type implicitly converts to
shared, and the shared U part separates out shared from the type so that you
can get the type without shared. That's then used for the alias to get the
type without shared. The way that the shared U part works comes from is
expressions:
https://dlang.org/spec/expression.html#is_expression
Eponymous templates get used in a variety of circumstances, not just with
aliases. For instance, they can be used with enums, e.g.
enum sizeOf(T) = T.sizeof;
which would be equivalent to
template sizeOf(T)
{
enum sizeOf = T.sizeof;
}
and they're used with function templates, e.g.
T identity(T)(T value)
{
return value;
}
is equivalant to
template identity(T)
{
T identity(T value)
{
return value;
}
}
- Jonathan M Davis
Re: Pointer to dlang spec for this alias construct?
On Monday, 17 June 2024 at 04:32:50 UTC, Andy Valencia wrote: In the alias: alias Unshared(T) = T; alias Unshared(T: shared U, U) = U; as used in: cast(Unshared!mytype)value turns a mytype with shared attribute into one without shared. I deduce the alias is using some sort of type matching and decomposition? I've read through the language spec, and didn't spot this mechanism. Can somebody tell me what section of the spec covers this very interesting mechanism? Thanks in advance, Andy not sure if it works the same with template arguments but here is the rules https://dlang.org/spec/expression.html#IsExpression
Pointer to dlang spec for this alias construct?
In the alias: alias Unshared(T) = T; alias Unshared(T: shared U, U) = U; as used in: cast(Unshared!mytype)value turns a mytype with shared attribute into one without shared. I deduce the alias is using some sort of type matching and decomposition? I've read through the language spec, and didn't spot this mechanism. Can somebody tell me what section of the spec covers this very interesting mechanism? Thanks in advance, Andy
