Need help with calling a list of functions

[Ali Çehreli]

On 11/03/2018 06:17 PM, Luigi wrote:
> I need to call a function that can create a function from an array of 
> functions and calls them in reverse order.  I am learning D any help 
> would be
> 
> 
> import std.stdio;
> import std.algorithm;
> import std.array : array;
> import std.range;
> 
> auto comp(T)(T function(T) [] list) pure {
>      auto backwards = retro(funs);
>      return <<HEre is where I think I need some help in calling the list 
> of functions with a delegate and fold or reduce>>;
> }
> 
> void main()
> {
>      auto fun = comp([(real x)=>a/3.0,(real x)=>x*x,(real x)=>x+1.0]);
>      writeln(fun(2.0));                // should print 3
> }
> 

Here is one that uses a loop:

import std.stdio;
import std.range : front;
import std.traits : ReturnType;

auto comp(Funcs...)(Funcs funcs) pure {
     alias R = ReturnType!(typeof([funcs].front));

     auto impl(R x) {
         foreach_reverse (func; funcs) {
             x = func(x);
         }
         return x;
     }

     return (double x) => impl(x);
}

void main()
{
     auto fun = comp((real x)=>x/3.0,(real x)=>x*x,(real x)=>x+1.0);
     assert(fun(2.0) == 3);
}

I used a variadic template parameter instead of an array parameter.

Ali