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

[Rikki Cattermole via Digitalmars-d-learn]

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.