[WIP] Explicit generic parameters to functions, the TypeGPU way

[iwoplaza]

Languages like C++, C#, Java and many more have a way to define generic parameters in functions that abstract over types and constant parameters that the function uses. It's similar to a regular function parameter, except that it operates on values that are known at compile-time, not a runtime. The Zig language makes that delineation even thinner, treating generic parameters the same as regular parameters, except prefixed with the comptime keyword.

We already have a way to do comptime specialization in TypeGPU via tgpu.slot and tgpu.derived, but explaining how to use them properly and using them in practice for generic programming can be a bit cumbersome. This was somewhat solved for parameter types with shellless functions that are inherently generic, but being generic over resources used by a function, or comptime parametrization is still only viable via the slot mechanism.

const geometricSeries = tgpu.comptime((iterations: number) => {
  // A function can have stable tinyest metadata
  // But the externals have to be recreated on every
  // invocation of `getPixel`. If we cached these
  // closures based on the externals, then we could
  // easily dedup the functions returned from here.
  // 
  // We could also more easily improve use in React.
  
  return (a: number, r: number) => {
    'use gpu';
    let result = 0;
    for (const i of tgpu.unroll(iterations)) {
      result += a * (r ** i);
    }
    return result;
  };
  // We can statically unwrap member accesses, unless the property is computed or a `$` (defensively).
  // We flatten the unwrapped members into a `cacheKey` member, which will be iterated over and compared
  // with other instances of the AST to dedup.
  // {
  //   externals: () => ({ tgpu: { unroll: tgpu.unroll }, iterations }),
  //   cacheKey: [tgpu.unroll, iterations],
  // }
  // 
});

const foo = () => {
  'use gpu';
  const almostTwo = geometricSeries(32)(1, 0.5);
};

[iwoplaza]

It's cool, but nah. We get automatic caching if we do this outside of the shader function.

+const geometricSeries32 = geometricSeries(32);
 const foo = () => {
   'use gpu';
-  const almostTwo = geometricSeries(32)(1, 0.5);
+  const almostTwo = geometricSeries32(1, 0.5);
 };