converting expression to delegate works only for variadic array?
Steven Schveighoffer
schveiguy at yahoo.com
Fri Jun 1 14:30:22 UTC 2018
On 6/1/18 10:08 AM, Gopan wrote:
> Hi,
> I created a test application (test.d) to learn delegates.
>
> import core.stdc.stdio;
>
> void Multi (int delegate()[] args ...)
> {
> foreach (exp; args)
> printf ("%d, ", exp() );
> printf ("\n");
> }
>
> void Single (int delegate() exp)
> {
> printf ("%d\n", exp());
> }
>
> void main()
> {
> int x = 5;
> Multi( {return 1;}, {return 2+x;} ); // outputs 1, 7 as expected.
> Multi( 1, 2+x); // same as above. produces same output.
> Multi( 3+x ); //produces output 8.
> Single( { return 3+x; } ); //produces output 8
> Single( 3+x ); // COMPILATION ERROR pasted towards bottom.
> // Good. But why does Multi(2+x) not have this issue?
>
> int delegate() exp = { return 4+x; };
> printf("%d\n", exp() );
> }
>
> test.d(22): Error: function test.Single(int delegate() exp) is not
> callable using argument types (int)
> test.d(22): cannot pass argument 3 + x of type int to parameter
> int delegate() exp
> make: *** [test.obj] Error 1
>
> I am happy with the statement Single(3+x) producing this compilation error.
>
> But why Multi(3+x) doesn't have this issue? For that, it is allowed as
> per 'Section 19.16.3.4 Lazy Variadic Functions' of the article
> https://dlang.org/spec/function.html#closures.
>
> What is the rationale behind not allowing it for the non-array signature?
These things were decided long ago, when D was just D1, and D2 didn't
even exist.
I think the rationale might go like this:
1. We have a lazy parameter, lazy int, which translates into a delegate
automatically.
2. Cool, we need a way to do this with variadics. But what does lazy
int[]... mean? Does it mean an array that's lazily calculated, or does
it mean an array of lazily calculated parameters?
3. lazy int[]... was decided to mean a lazily calculated int array (the
whole array is created whenever it is used).
4. int delegate()[]... was decided to mean an array of lazily passed ints.
IMO, lazy X and X delegate() should be interchangeable API-wise and
exactly the same. You should be able to call either one with an X or a
delegate. This makes the most sense and is the most useful.
-Steve
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