It is challenging to deal with functional languages, especially non-strict functional languages, without GC. You can do it to some degree, but it requires some clever and inconvenient limitations on the language. Lifetimes and linear types are two approaches that can help. Clash and lambda-ccc can convert non-monomorphically-recursive Haskell code to a fixed-memory implementation (not even a stack, let alone a heap). Closures, unbounded recursion, and all those other things that make functional programming so convenient often require GC.