digitalmars.D.learn - Differences between lambda function and regular functions in higher
- steve (23/23) Feb 21 2022 I am trying to implement a simple map function. I found code to
- partypooper (7/30) Feb 21 2022 I guess because your function parameter is actually a pointer to
- Stanislav Blinov (46/58) Feb 21 2022 As partypooper says, with that singature it'll only work if you
- steve (3/3) Feb 21 2022 thanks a lot both! Yes I'm aware that map exists already. This
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)); } ```
Feb 21 2022
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(×two, my_array)); ```
Feb 21 2022
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(×two, 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.
Feb 21 2022
thanks a lot both! Yes I'm aware that map exists already. This was didactic. I had tried to find out whether lambdas generate function pointers but also couldn't figure that one out :D
Feb 21 2022