memoize (is this a fix for overloading?)

[Max Samukha]

On 01/05/2011 12:24 PM, Lars T. Kyllingstad wrote:
> On Wed, 05 Jan 2011 12:07:50 +0200, Max Samukha wrote:
>
>> On 01/05/2011 11:21 AM, Lars T. Kyllingstad wrote:
>>> On Tue, 04 Jan 2011 17:06:45 -0600, Andrei Alexandrescu wrote:
>>>
>>>> On 1/4/11 4:49 PM, %u wrote:
>>>>>> There's still the risk of keeping multiple hashes. Consider:
>>>>>
>>>>>> ulong fun(ulong n) { ... }
>>>>>> alias memoize!fun mfun;
>>>>>
>>>>>> mfun(5); // creates hash ulong[int]
>>>>>> mfun(5u); // creates hash ulong[uint] mfun('5'); // creates hash
>>>>>> ulong[char]
>>>>>
>>>>>
>>>>> Ohhh I see... so you're basically looking for a compile-time version
>>>>> of overload resolution, right? Because things seem to be getting
>>>>> complicated very quickly.
>>>>
>>>> It's not that complicated; we'll be able to achieve it somehow.
>>>> Overloads are compile-time entities so they should be easily
>>>> inspectable.
>>>
>>> Since this is likely to be useful in other cases as well, maybe it
>>> would be better to make a general template that selects an overload?
>>> So you'd use it like this:
>>>
>>>     alias memoize!(selectOverload!(sqrt, double)) msqrt;
>>>
>>> A first step towards this would be to make __traits(getOverloads) work
>>> with module-level functions.  Currently it only works for member
>>> functions.  Also it would probably be better if it accepted a function
>>> alias instead of a string containing the function name.
>>
>> __traits(getOverloads) does work with module level functions if you pass
>> a module alias to it.
>
> Cool!  I didn't know that.  Here's an updated version, where
> selectOverload works with both module-level and member functions:
>
>
> module test;
> import std.traits, std.stdio;
>
> void foo(int i, double d) { writeln("Overload 1"); }
> void foo(double d, int i) { writeln("Overload 2"); }
>
> void main()
> {
>      alias selectOverload!(test, "foo", int, double) bar;
>      bar(1, 3.0);    // Prints "Overload 1"
>
>      alias selectOverload!(test, "foo", double, int) baz;
>      baz(4.0, 2);    // Prints "Overload 2"
> }
>
>
>
> template selectOverload(alias parent, string fun, Params...)
> {
>      alias selectOverloadImpl!(Params.length, Params,
>          __traits(getOverloads, parent, fun)) selectOverload;
> }
>
>
> template selectOverloadImpl(size_t lenP, A...)
> {
>      static assert (A.length>  lenP, "No overload matches");
>      static if (equalTuples!(lenP,
>          A[0 .. lenP],
>          ParameterTypeTuple!(A[lenP])))
>      {
>          alias A[lenP] selectOverloadImpl;
>      }
>      else
>      {
>          alias selectOverloadImpl!(lenP, A[0 .. lenP], A[lenP+1 .. $])
>              selectOverloadImpl;
>      }
> }
>
>
> template equalTuples(size_t len, T...)
> {
>      static if (len == 0&&  T.length == 0)
>          enum equalTuples = true;
>      else static if (T.length != len*2)
>          enum equalTuples = false;
>      else static if (!is (T[0] == T[len]))
>          enum equalTuples = false;
>      else
>          enum equalTuples =
>              equalTuples!(len-1, T[1 .. len], T[len+1 .. $]);
> }

Nice! Though I think that overloads should be selected not by type 
equality but by regular overload resolution rules. That is the template 
should take not parameter but argument types.