The standard library (Phobos) is open source. You can find the implementation of memoize at https://github.com/D-Programming-Language/phobos/blob/6cd2e4...

memoize is a template (http://dlang.org/template.html). In this case, a template is used to create a "version" of "memoize" with the right parameters and return value (i.e., `fn` and `memoize!fn` are the same type).

Each of these "versions" has a hash table from argument tuples to results. I believe the hash is computed by calling a toHash function for each item in the tuple.

Tuples and type tuples (http://dlang.org/tuple.html) in D are really quite powerful.