I'm curious how D implements memoizing an arbitrary function. To memoize something, you need to be able to check if it's being called with the same arguments. But to do that, you need a definition of "same" (i.e., an equivalence relation). For example, if the argument is a dictionary, is it considered the same argument only if the pointer is the same, or if the keys and values are the same, assuming a recursive definition of "same"? Also, you need a way to store computed values, using the arguments as keys, which for efficiency would require either an ordering (for a tree) or hashing (for a hash table) on the arguments. Multiple arguments would further complicate things. How does this get automated? And for some things, memoization wouldn't make sense (for example, with function arguments in higher-order functions). Sensible defaults would take care of most of these things, but it makes me wonder how they chose to address these things. And is the memo recreated each time the (outer) function is called, or is it stored for the duration of the program? Also, how does the type system play with this? What's the type of `memoize!`?