A system I work with gives all keys a string value of “Not_set” when the key is intended to be unset. The team decided to put this in because of a connection with a different, legacy system, whose developers (somehow) could not distinguish between a key being missing or being present but with a null value. So now every team that integrates with this system has to deal with these unset values.
Of course, it’s up to individual developers to never forget to set a key to “Not_Set”. Also, they forgot to standardise capitalisation and such so there are all sorts of variations “NOT_SET”, “Not_set”, “NotSet”, etc. floating around the API responses. Also null is still a possible value you need to handle as well, though what it means is context dependent (usually it means someone fucked up).
The basic problem is that identifiers can be either types or variables, and without a keyword letting you know what kind of statement you’re dealing with, there’s no way of knowing without a complete identifier table. For example, what does this mean:
If foo is a type, that is a pointer declaration. But if it’s a variable, that’s a multiplication expression. Here’s another simple one:
foo(bar);
Depending on how foo is defined, that could be a function call or a declaration of a variable bar of type foo, with some meaningless parentheses thrown in.
When you mix things together it gets even more crazy. Check this example from this article:
foo(*bar)();
Is bar a pointer to a function returning foo, or is foo a function that takes a bar and returns a function pointer?
let
andfn
keywords solve a lot of these ambiguity problems because they let the parser know what kind of statement it’s looking at, so it can know whether identifiers in certain positions refer to types or variables. That makes parsing easier to write and helps give nicer error messages.