On Thu, Apr 21, 2022 at 09:02:47PM +0000, JG via Digitalmars-d-learn wrote:
> Hi,
>
> Could someone possibly help me to understand why the commented line
> doesn't compile?
>
>
> ```d
> import std;
>
> struct MapResult(R,F)
> {
> R r;
> const F f;
> auto empty() { return r.empty; }
> auto front() { return f(r.front); }
> void popFront() { r.popFront; }
> auto save() { return typeof(this)(r.save,f); }
> }
>
> auto myMap(alias f, R)(R r) {
> return MapResult!(R,typeof(f))(r,f);
> }
>
>
> void main()
> {
> int function(int) f = x=>2*x;
> iota(10).myMap!f.writeln;
> //iota(10).myMap!(x=>2*x).writeln; <--- Why doesn't this compile?
>
> }
> ```
Check the compiler's error message carefully:
/tmp/test.d(6): Error: variable `test.MapResult!(Result, void).MapResult.f`
variables cannot be of type `void`
^^^
The error comes from line 6, which is:
> const F f;
A lambda literal like `x=>2*x` is a template, so you cannot store it in
a variable. You need to either explicitly wrap it in a delegate, or
forward the alias to your struct as a template parameter, e.g.:
struct MapResult(alias f, R)
{
... // remove the line `const F f;`
}
auto myMap(alias f, R)(R r) {
return MapResult!(f, R)(r);
}
T
--
Truth, Sir, is a cow which will give [skeptics] no more milk, and so they are
gone to milk the bull. -- Sam. Johnson
