I prefer an expressive API to a smart one. Which is to say that I wouldn’t mind specifying the expected “history” up front when allocating. I’ve been thinking

I prefer an expressive API to a smart one. Which is to say that I wouldn’t mind specifying the expected “history” up front when allocating. I’ve been thinking of writing a library for “roles”, which would let you describe how a value is expected to change over time, and thereby encode some of the intent about what a variable or bit of code is for, with the happy side effect of helping the runtime system select fast paths. For example, the “accumulator” role would be for values that are expected to have a property that grows monotonically, like the size of a vector or the value of a counter. For a vector, you might additionally encode whether its growth should be linear or exponential, what its minimum and maximum sizes should be, and various growth factors. Violating a role would not be an error, though it might be slow, and in debug mode you might get a diagnostic message to help you adjust the roles in your program. Some role-like features are already present in languages, libraries, and hardware, such as “restrict” to encode the expectation of non-aliased buffers, “__builtin_expect” to encode likelihood, and “__builtin_prefetch” to encode the intent to access a value soon.