How can I express the type '(int) => int' where it is a function or a delegate

[Puming via Digitalmars-d-learn]

On Wednesday, 16 July 2014 at 04:10:13 UTC, Rikki Cattermole 
wrote:
> On 16/07/2014 3:50 p.m., Puming wrote:
>> I'd like to have a Command class, where their is a name and a 
>> handler
>> field:
>>
>> ```d
>> class Command
>> {
>>   string name;
>>   string delegate(string[]) handler;
>> }
>> ```
>>
>> this is ok, but sometimes I want the handler also accept a 
>> function
>> (lambdas are init to functions if no capture of outer scope 
>> variables
>> are present), but it can't.
>>
>> So I'd like to generalize the Command to a template, the best 
>> I've got
>> sofar:
>>
>> ```d
>>
>> alias string delegate(string[]) HandlerDele;
>> alias string function(string[]) HandlerFunc;
>>
>> class Command(T) if (is (T HandlerDele) || is (T HandlerFunc))
>> {
>>     immutable {
>>         string name;
>>         T handler;
>>     }
>>
>>     this(string name, T handler)
>>     {
>>         this.name = name;
>>         this.handler = handler;
>>     }
>>
>> }
>>
>> void main()
>> {
>>
>>     HandlerFunc f = xs => xs[0]; // just a test
>>     auto cmd = new Command!HandlerFunc("echo", f);
>> }
>> ```
>>
>> I've got several questions about this:
>>
>> 1. I cant ignore `HandlerFunc` when initiating cmd:
>>
>> ```d
>> auto cmd = new Command("echo", f); // Error: class
>> dshell.command.Command(T) if (is(T HandlerDele) || is(T 
>> HandlerFunc)) is
>> used as a type
>> ```
>>
>> Can DMD automatically infer the type here?
>>
>> 2. Is this the right way to do this?
>>
>> 3. I'd like a unified description of `a function pointer or a 
>> delegate`,
>> and from the experience of lambda, it seems the syntax of 
>> lamdba is
>> really useful here, if we have that, then instead of:
>>
>> ```d
>> void execute(T)(Context cxt, T handler) if (is (T HandlerFunc) 
>> || is (T
>> HandlerDele))
>> {
>>   //...
>> }
>>
>> we could define a function that accepts a function/delegate 
>> like this:
>>
>> ```d
>>
>> void execute(T : string[] => string)(Context cxt, T handler)
>> {
>>   //...
>> }
>>
>> // in main
>> ctx.execute(xs => xs[0]);
>
> Or using std.functional toDelegate you could convert the 
> function into a delegate.
>
> class Command {
>     string name;
>     string delegate(string[]) handler;
>
>     this(string name, string delegate(string[]) handler) {
>         this.name = name;
>         this.handler = handler;
>     }
>
>     this(string name, string function(string[]) handler) {
>         import std.functional : toDelegate;
>         this.name = name;
>         this.handler = toDelegate(handler);
>     }
> }
>
> Just keep in mind, you can't go the opposite way.

Thanks. I wonder if functions could implicitly convert to 
delegates...but toDelegate is OK.


Also, after another dig into the language docs, I found:


> The .ptr property of a delegate will return the frame pointer 
> value as a void*.

> The .funcptr property of a delegate will return the function 
> pointer value as a function type.

> Future directions: Function pointers and delegates may merge 
> into a common syntax and be interchangeable with each other.

Wonder how that would happen.