Differences between lambda function and regular functions in higher order functions

[partypooper]

On Monday, 21 February 2022 at 10:04:16 UTC, steve wrote:
> I am trying to implement a simple map function. I found code to 
> do this in another post but it only seems to work with lambda 
> functions and I do not understand why. Any help would be 
> greatly appreciated
>
> ```
> import std.stdio;
>
> T[] map_vals(T,S)(scope T function(S) f, S[] a){
>     auto b = new T[a.length];
>     foreach(i,x;a) b[i] = f(x);
>     return b;
> }
>
>
> auto timestwo(float x) {
>     return 2*x;
>     }
>
> void main(){
>     float[] my_array = [1., 2., 3.];
>     auto ff = (float x)=>2*x;
>
>     // This works
>     writeln(map_vals(ff, my_array));
>
>     // this does not
>     // writeln(map_vals(timestwo, my_array));
> }
> ```

I guess because your function parameter is actually a pointer to 
a function. ff is a pointer to anonymous function. timestwo is 
not. This should work

```d
writeln(map_vals(&timestwo, my_array));
```