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Wow, I honestly wonder if this was caused by mixing up “functor” in the C++ and Haskell senses… To expand on that, for those in the audience: In Haskell, a functor is a type constructor (like “list”, “optional”, “future”, “I/O request”, &c.) with a way to map a function over it, covariantly, in a way that preserves its structure—i.e. without changing the shape of the container or structure of the action represented by the constructor, just the contained elements or produced result. This is based on the more general notion of a functor in mathematics, which is a mapping between categories. The Haskell version is much more constrained, though: it only maps between Haskell types, and it’s parametric (iow completely generic), not just any old mapping. While in C++, a functor is a completely different thing: an object that can be called like a function. It’s thus equivalent to a closure, where the object fields are the captured values. And that sounds like the description being used here. |