This is a nice example of why one should parse, not validate. If every function that requires some kind of permission takes that permission as an argument, say

This is a nice example of why one should parse, not validate. If every function that requires some kind of permission takes that permission as an argument, say (pseudocode) void doFoo(PermissionToDoFoo permission, ...){...} and then, the only way to call it is through something like from request import getAuth, respond \\ Maybe<AuthenticationData> getAuth(Request request) \\ void respond(String response) from permissions import askForPermissionToDoFoo \\ Maybe<PermissionToDoFoo> askForPermissionToDoFoo(AuthenticationData auth) response = try auth <- getAuth(request) permission <- askForPermissionToDoFoo(auth) doFoo(permission) "Success!" fail "Oopsie!" respond(response) It becomes impossible to represent the invalid state of doing Foo without permission.