How to reuse functions

[John Colvin via Digitalmars-d-learn]

On Friday, 1 May 2015 at 03:34:53 UTC, Luigi wrote:
> Hi everybody.
>
> I am tring to use a function where its parameter is another 
> function, and at the same time are both already made - they 
> cannot be modified - and the second one has to be conditioned 
> before to be passed as argument.
>
> Let's say I have these function and I cannot modify:
>
> -jac(+d) that works on function and return real
>
> real jac(real function(real) fun, real x) {return d(fun, x);}
> real d(real function(real) fun, real x) {return fun(x);}
>
> -F1 that works on two reals and return real
>
> real F1(real a, real b) {return a + b;}
>
> So I need a way to condition F1 fixing b and then pass it to 
> jac as argument.
>
> To do that I've created a delegate 'simp' conditionig F1:
>
> real delegate(real) simp(real function(real, real) f, real x) {
> 	real _simp(real z) {
> 		return f(z, x);
> 		}
> 	return &_simp;
> 	}
> (here 'simp' fixes b at x).
>
> My main is:
>
> void main() {
> 	real x_n = 1;
> 	real x_m = -1;
> 	real delegate(real) s_n = simp(&F1, x_n);
> 	//auto J = jac(s_n, x_m); //Error: function app.jac (real 
> function(real) fun, real x) is not callable using argument 
> types (real delegate(real), real)
> 	}
>
> the code fails because jac expect as argument a function but I 
> found only a delegate to obtain a simplified function without 
> any touch at already made functions jac, d and F1.
>
> There is a clean way to make it possible? I mean: without to 
> touch (neither rewrite) jac, d and F1, obtain simplied F1 to 
> pass to jac.
>
> Thaks in advance to anyone could help me.
> Luigi

A combination of std.functional.reverseArgs and 
std.functional.partial might help. It's unfortunately we don't 
have a version that can set an arbitrary argument, only the first 
one; It would be an easy improvement to make.