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digitalmars.D.learn - Passing functions to functionals

reply "Lars T. Kyllingstad" <public kyllingen.NOSPAMnet> writes:
In my library I have a lot of functionals (functions that take other 
functions as parameters).  Here is an example that shows the style I use 
to define them:

    // Example: Evaluate the function/delegate/functor f at x.
    auto eval(F, X)(F f, X x) { return f(x); }

    // Test
    void main()
    {
        int add2(int i) { return i + 2; }
        assert (eval(&add2, 1) == 3);
    }

In other words, the function is passed as a run-time parameter.  I've 
seen this (or similar) style used in Phobos, but there, I've also noted 
that functions are sometimes passed as template alias parameters:

    // Same as above, using template alias parameter.
    auto eval(alias f, X)(X x) { return f(x); }

    // Test
    void main()
    {
        int add2(int i) { return i + 2; }
        assert (eval!add2(1) == 3);
    }

I'd be grateful if people would share their knowledge of the pros and 
cons of each method.  For instance, are there any situations where 
template alias parameters don't work?


Thanks,

-Lars
Nov 30 2010
next sibling parent reply Dmitry Olshansky <dmitry.olsh gmail.com> writes:
On 30.11.2010 14:59, Lars T. Kyllingstad wrote:
 In my library I have a lot of functionals (functions that take other
 functions as parameters).  Here is an example that shows the style I use
 to define them:

      // Example: Evaluate the function/delegate/functor f at x.
      auto eval(F, X)(F f, X x) { return f(x); }

      // Test
      void main()
      {
          int add2(int i) { return i + 2; }
          assert (eval(&add2, 1) == 3);
      }

 In other words, the function is passed as a run-time parameter.  I've
 seen this (or similar) style used in Phobos, but there, I've also noted
 that functions are sometimes passed as template alias parameters:

      // Same as above, using template alias parameter.
      auto eval(alias f, X)(X x) { return f(x); }

      // Test
      void main()
      {
          int add2(int i) { return i + 2; }
          assert (eval!add2(1) == 3);
      }

 I'd be grateful if people would share their knowledge of the pros and
 cons of each method.  For instance, are there any situations where
 template alias parameters don't work?


 Thanks,

 -Lars

on what you can't do: import std.stdio; auto eval(F, X)(F f, X x) { return f(x); } auto eval2(alias f,X)(X x){ return f(x); } auto summator(int k){ int f(int val){ return k + val; } return &f; } // Test void main() { int add2(int i) { return i + 2; } int traceAdd2(int i){ writeln(i," --> ",i+2); return i+2; } int delegate(int) getAdd2(){ writeln("Getting add2"); return &add2; } assert(eval(&add2, 1) == 3); assert(eval2!add2(1) == 3); assert(eval(summator(2),1) == 3); //Next one fails with //Error: closures are not yet supported in CTFE //Error: cannot evaluate summator(2) at compile time //assert(eval2!(summator(2))(1) == 3); assert(eval(&traceAdd2,1) == 3); assert(eval2!traceAdd2(1) == 3); //side effect in call is no problem assert(eval(getAdd2,1) == 3); //Next one fails with //Error: cannot evaluate writeln("Getting add2") at compile time //Error: cannot evaluate getAdd2() at compile time //assert(eval2!(getAdd2())(1) == 3); } Well, once the limitations of CTFE are dealt with, the only problem alias param not capable of would be user input parameters, and side-effects during delegate/function construction. IMHO this can pose a problem with some algorithms in std.algorithm, like _filter_ing on user provided criteria currently impossible (since predicate parameter to filter has to be known at compile time). One soultion might be is somehow provide alternative versions of the with the first version of syntax. A distinctive feature of alias parameters - they are not passed as arguments at all, as the (stripped) asm suggests: for eval2!add2(1): _TEXT:00405AB4 enter 4, 0 _TEXT:00405AB8 mov [ebp+var_4], eax _TEXT:00405ABB push [ebp+arg_0] _TEXT:00405ABE mov eax, [ebp+var_4] _TEXT:00405AC1 call test_e main add2; direct call, compiler knows what the alias is _TEXT:00405AC6 leave _TEXT:00405AC7 retn 4 eval(&add2, 1): _TEXT:00405A98 var_4 = dword ptr -4 _TEXT:00405A98 arg_0 = dword ptr 8 _TEXT:00405A98 arg_4 = dword ptr 0Ch _TEXT:00405A98 _TEXT:00405A98 enter 4, 0 _TEXT:00405A9C push ebx _TEXT:00405A9D mov [ebp+var_4], eax _TEXT:00405AA0 push [ebp+var_4] _TEXT:00405AA3 mov eax, [ebp+arg_0] _TEXT:00405AA6 mov edx, [ebp+arg_4] _TEXT:00405AA9 mov ebx, [ebp+arg_0] _TEXT:00405AAC call edx ;<---- note the indirect call _TEXT:00405AAE pop ebx _TEXT:00405AAF leave _TEXT:00405AB0 retn 8 That's as far as I can get for now, there is still room for investigation. -- Dmitry Olshansky
Nov 30 2010
parent "Lars T. Kyllingstad" <public kyllingen.NOSPAMnet> writes:
On Tue, 30 Nov 2010 18:49:56 +0300, Dmitry Olshansky wrote:

 On 30.11.2010 14:59, Lars T. Kyllingstad wrote:
 In my library I have a lot of functionals (functions that take other
 functions as parameters).  Here is an example that shows the style I
 use to define them:

      // Example: Evaluate the function/delegate/functor f at x. auto
      eval(F, X)(F f, X x) { return f(x); }

      // Test
      void main()
      {
          int add2(int i) { return i + 2; }
          assert (eval(&add2, 1) == 3);
      }

 In other words, the function is passed as a run-time parameter.  I've
 seen this (or similar) style used in Phobos, but there, I've also noted
 that functions are sometimes passed as template alias parameters:

      // Same as above, using template alias parameter. auto eval(alias
      f, X)(X x) { return f(x); }

      // Test
      void main()
      {
          int add2(int i) { return i + 2; }
          assert (eval!add2(1) == 3);
      }

 I'd be grateful if people would share their knowledge of the pros and
 cons of each method.  For instance, are there any situations where
 template alias parameters don't work?


 Thanks,

 -Lars

on what you can't do: import std.stdio; auto eval(F, X)(F f, X x) { return f(x); } auto eval2(alias f,X)(X x){ return f(x); } auto summator(int k){ int f(int val){ return k + val; } return &f; } // Test void main() { int add2(int i) { return i + 2; } int traceAdd2(int i){ writeln(i," --> ",i+2); return i+2; } int delegate(int) getAdd2(){ writeln("Getting add2"); return &add2; } assert(eval(&add2, 1) == 3); assert(eval2!add2(1) == 3); assert(eval(summator(2),1) == 3); //Next one fails with //Error: closures are not yet supported in CTFE //Error: cannot evaluate summator(2) at compile time //assert(eval2!(summator(2))(1) == 3); assert(eval(&traceAdd2,1) == 3); assert(eval2!traceAdd2(1) == 3); //side effect in call is no problem assert(eval(getAdd2,1) == 3); //Next one fails with //Error: cannot evaluate writeln("Getting add2") at compile time //Error: cannot evaluate getAdd2() at compile time //assert(eval2!(getAdd2())(1) == 3); }

That's a very good point, and pretty much settles it for me. I'll stay away from alias parameters, then. Thanks! -Lars
Dec 01 2010
prev sibling parent Jesse Phillips <jessekphillips+D gmail.com> writes:
Lars T. Kyllingstad Wrote:

     // Same as above, using template alias parameter.
     auto eval(alias f, X)(X x) { return f(x); }
 
     // Test
     void main()
     {
         int add2(int i) { return i + 2; }
         assert (eval!add2(1) == 3);
     }
 
 I'd be grateful if people would share their knowledge of the pros and 
 cons of each method.  For instance, are there any situations where 
 template alias parameters don't work?
 
 
 Thanks,
 
 -Lars

Well, with using an alias parameter then std.functional.unaryFun/binaryFun is usable so you could use the eval!"a+2"(5) syntax.
Nov 30 2010