> How could you ever (in a non-trivial case) avoid local state mutation? It seems like any function which takes a collection and returns a collection would have to maintain some local state.
Somewhere underneath there will need to be something maintaining state, but it won't have to be local state in the function (in a pure language, typically it will be within a built-in with a pure interface.) FP isn't about changing the fact that computers operate by mutating state, but to isolate such mutations (and other side effects) behind pure interfaces, so that risk and difficulties associated with effectful code are isolated to, ideally, extremely well understood pieces of infrastructure code rather than permeating large codebases.
Somewhere underneath there will need to be something maintaining state, but it won't have to be local state in the function (in a pure language, typically it will be within a built-in with a pure interface.) FP isn't about changing the fact that computers operate by mutating state, but to isolate such mutations (and other side effects) behind pure interfaces, so that risk and difficulties associated with effectful code are isolated to, ideally, extremely well understood pieces of infrastructure code rather than permeating large codebases.