So many questions left unasked! In particular they have "recursion disabled by default" but "we have all the usual recursive combinators". Does this mean that they can't explicitly write recursion buy may achieve recursive effects only through the combinators? I'd like to know more about this. Is it as expressive as recursion only through "properly optimised" combinators?
By combinators they mean things like folding/reducing over lists, i.e. combinators/higher order functions for operating on potentially infinite structures.
It's not as expressive as raw recursion since many uses of recursion are hard to express through combinators, albeit a great deal of research has been done in this area (look up recursion schemes).
The reason is that we don't have a general solution for tail-call optimisation for computations made of a mixture of Mu code and C++ primitives.
We have higher order functions defined in C++ that can be called from Mu, and can call back Mu functions passed to them. This back and forth between languages has occurred naturally in recursive definitions, and we don't have a way of reusing stack frames between the Mu interpreter and the native C++ code for these higher-order functions.
We decided a long time ago to disable recursion by default, encouraging programmers to use recursion operators. We do however have a Mu language extension that enables recursion, which is used when the recursion depth is known to be bounded.
It has been argued that unrestricted recursion is like GOTO. I have some limited experience with "recursion schemes", they're quite expressive. But I feel like it's trying to give every possible pattern of recursion a name, to the point that it's almost becoming a joke - e.g. "zygohistomorphic prepromorphism".
