So, this is something that we've considered for a while now, after all these years writing production code in Nim, we long for Python's lack of static checking; in the end there is always a Nim code snippet that tests the part of the Nim compiler we touched anyway, the static type system doesn't speed up Nim's core development. Python is simply more productive to work in, especially considering its ecosystem. For example, we want to add machine learning to the Nim compiler, if the Nim compiler were written in Python, it would be as simple as `import machinelearning`.
However, it is well known that Python code can sometimes be slow if it's CPU bound. So these CPU bound parts of the Nim compiler will be written as Python extension modules in C. Writing a product in two vastly different programming languages allows us to use each language for the part where it shines, C for speed, Python for the glue code. The perfomance critical parts inside the Nim compiler are probably: The lexer, the parser, the semantic checker, the overloading resolution mechanism, the VM for macro evaluations and the code generators. Thankfully C is a well understood language with a friction-less type system, for example, I think we can get away with modelling the Nim AST in C via a single `void*` type. We have been working on this in the last couple of months secretly and these improvements will be part of the upcoming version 1.2 release.
