Is "Lisp-1 vs Lisp-2" relevant in a language with static types?

Question

(This is a CS-theory type of question; I hope that's acceptable.)

The "Lisp-1 vs Lisp-2" debate is about whether the namespace of functions should be distinct from the namespace of all other variables, and it's relevant in dynamically typed languages that allow the programmer to pass around functions as values. Lisp-1 languages (such as Scheme) have one namespace, so you can't have both a function named f and also an integer named f (one would shadow the other, just like two integers named f). Lisp-2 languages (such as Common Lisp) have two namespaces, so you can have both f variables, but you have to specify which one you mean with special syntax (#'f is the function and f is the integer).

It seems to me that the main technical problem, the need to disambiguate the function from the integer, is not an issue if the language is also statically typed (unlike most Lisps). For instance, if a sort function requires a list and a less-than function as an explicit signature,

def sort[X](list: List[X], lessThan: Function[X, Boolean])    // Scala syntax; had to pick something

then it wouldn't matter if the functions and everything else are in the same namespace or not. sort(mylist, myless) would only pass a type check if myless is a function--- no special syntax needed. Some people argue that one namespace is more aesthetically pleasing than two namespaces, but I'd like to focus on technical issues.

Is there anything that two namespaces would make more difficult or more prone to error (or conversely for one namespace), assuming that the language in question is statically typed?

(I'm thinking about this in the context of a domain specific language that I'm working on, and I want to make sure that I don't run into problems down the road. It would be easier to implement with two namespaces (Lisp-2), and since it's statically typed, there's no need for the equivalent of #'f. I asked the question in general because I want to hear general points and perhaps become aware of questions that I don't yet know to ask.)

Answer 1

One very common objection to multiple namespaces is that it complicates the formal semantics to have an arbitrary number of namespaces. One namespace makes things simple. The next simplest, so I'm told (I don't write these things), is an infinite number of namespaces--something I tried to do once and only got halfway through (see here if curious, though I think it's not what you're asking for in this case). It's when you limit it to a finite number of namespaces that a formal semantics gets messy, or so I'm told. And certainly it makes any kind of compiler or interpreter a bit more complex, too. This objection straddles between aesthetic and technical in that it's not an objection based on technical difficulty per se, as no super-human intelligence is required to do multiple namespaces, just a lot of extra manual labor, but rather an objection that doing a more complex semantics means more code, more special cases, more chances for error, etc. Personally, I'm not swayed by such arguments, I merely point them out since you ask. I think you'll find such arguments aren't fatal and that it's fine to proceed on what you're doing and see where it leads. I care much more about programmer/end-user experiences than implementor difficulty. But I mention this argument because other people I respect seem to think this is a big deal and I believe it's the correct answer to your question about difficulty and error-prone-ness.

Note, by the way, that when Gabriel and I wrote Technical Issues of Separation in Function Cells and Value Cells, I needed words to help me avoid saying "Scheme-like" and "Lisp-like" because people had unrelated reasons for preferring Scheme that had nothing to do with namespacing. Using the terms "Lisp1" and "Lisp2" allowed me keep the paper from becoming a Scheme vs. Lisp debate, and to focus readers narrowly on the matter of namespace. In the end, ANSI Common Lisp ended up with at least 4 namespaces (functions, values, go tags, block tags) or maybe 5 if you count types (which are sort of like a namespace in some ways but not others), but in any case not 2. So strictly it would be a Lisp4 or Lisp5. But either way it still horrifies those who prefer a single namespace because any arbitrary finite number bigger than one tends to be unacceptable to them.

My personal recommendation is that you should design the language according to your personal sense of what's right. For some languages, one namespace is fine. For others not. Just don't do it because someone tells you that either way has to be the right way--it's really legitimately your choice to make.

Some argue that one namespace is conceptually simpler, but I think that depends on the notion of simplicity. Some say smaller is simpler (notationally or implementationally). I claim that what's conceptually simplest is what's most isomorphic to how your brain thinks about things, and that your brain may not always be thinking "small" for "simple". I'd cite as at least one example that every human language in existence has multiple definitions for the same word, almost all resolved by context (of which your type inferencing might be one example of contextual information), suggesting that wetware is designed to disambiguate such things and that your brain is wasting some of its natural capacity if you don't let it care about such things. Maybe indeed this contextual inferencing increases the complexity of your language implementation, but languages are implemented only once, and are used many times, so there's every reason to optimize the end user experience, not the implementor experience. It's an investment that pays off later.

Insist on thinking about things how you want. Worry more about making your language have a good feel and about whether what you want to do is implementable at all, even if it does require work and extra checking of the code. No language design decision is the right one for every language--languages are ecologies and what matters is that language elements work well with one another, not that language elements match other languages. Indeed, if languages were going to be all the same, it would be pointless to have multiple languages.