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

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.