Differences between lambda function and regular functions in higher order functions

[Stanislav Blinov]

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;
> }
> ```


As partypooper says, with that singature it'll only work if you 
pass function pointer (whereas a lambda converts to one). 
Alternatively (and how it is typically done in i.e. D's standard 
library), you can pass your callable as a compile-time argument. 
This also has an advantage of supporting UFCS, as shown in this 
example:

```d
import std.stdio;

// original, needs a function pointer
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;
}

// Takes the callable as a compile-time argument
auto map_vals(alias f,S)(S[] a)
if (is(typeof(f(a[0]))))
{
     alias T = typeof(f(a[0]));
     auto b = new T[a.length];
     foreach (i, ref 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 works with pointer to timestwo
     writeln(map_vals(&timestwo, my_array));

     // and this works by just passing the symbol name,
     // also note UFCS syntax:
     my_array.map_vals!timestwo.writeln;

     // as does this:
     my_array.map_vals!ff.writeln;

     // and this:
     my_array.map_vals!(x => 2*x).writeln;
}
```

Note that `map` already exists in Phobos 
(https://dlang.org/phobos/std_algorithm_iteration.html#map), and 
that one makes a lazy range and doesn't allocate.