>The normal definition of functional programming, in which not all data structures used in imperative programming are idiomatic I can't find any reference anyw

>The normal definition of functional programming, in which not all data structures used in imperative programming are idiomatic I can't find any reference anywhere to any sort of definition like that. Or even one that mentions restricting data structures or memory layout at all. The only "normal" definitions I can find are: "functions are first class" which is a pretty weak definition and doesn't really exclude much these days, and: "the return values of functions depend only on the functions arguments" which is what everyone in the haskell world thinks of as "functional programming". Neither of these definitions exclude arrays. >For starters, mutable data structures would be considered not purely functional A data structure can not be functional or imperative. Only what you do with it. Mutation is perfectly fine if it is localized (remember the function's return value needs to only depend on its arguments, nothing more).