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Comment by BoiledCabbage | Hacker News Reader

> Sadly this is not the case with Scheme and it makes for very unergonomic code,

How so? If car of nil returns nil, then how does a caller distinguish between a value of nil and a container/list containing nil?

The only way is they can check to see if it's a cons pair or not? So if you have to check if it's a cons pair then you're doing the same thing as in scheme right?

I may be missing something, but isn't it effectively the same amount of work just potentially? Need to check for nil and need to check if it's a pair?

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> how does a caller distinguish between a value of nil and a container/list containing nil

Very easily; but the point is that it's very often easy to design things so that the caller doesn't have to care.

For instance, lookup in an associative list can just be (cdr (assoc key alist)).

If the key is not found, assoc returns nil, and so cdr returns nil.

Right, so when we use this shortcut, we have an ambiguity: does the list actually have that key, but associated with the value nil? Or does it not have the key.

Believe it or not, we can design the data representation very easily such that we don't care about the difference between these two cases; we just say we don't have nil as a value; a key with a value nil is as good as a missing key.

This situation is very often acceptable. Because, in fact, data structures are very often heavily restrained in what data types they contain. Whenever we assert that, say, a dictionary has values that are, say, strings, there we have it: values may not be nil because nil is not a string. And so the ambiguity is gone.

A nice situation occurs when keys are associated with lists of values. A key may exist, but be associated with an empty list (which is nil!). Or it may not exist. We can set things up so that we don't care about distinguishing these two. If key K doesn't exist then K is not associated with a list of items, which is practically the same as being associated with an empty list of items. If we split hairs, it isn't, but in a practical application things can be arranged so it doesn't matter.

> How so? If car of nil returns nil, then how does a caller distinguish between a value of nil and a container/list containing nil?

How about this?

  CL-USER> (null nil)
  T
  CL-USER> (null '(nil))
  NIL
  CL-USER>

I think that's my point. You still need a separate call to distinguish the nil rom the list of nil case.

At that point, if you're making the two calls how is LISP's behavior any more ergonomic than Scheme. I'm not saying it's not possible, I just don't see it.

Can you show code between the two and how one is much worse than the other?

cons is an adt and fundamental building block used to build lists (which is a builtin datatype) it's also used to build other data types. the property we're discussing is useful when you're operating on those other data types, rather than lists. when you're designing those other data types you have to be aware that null can be both the absence of value and a value, so you design those other data types appropriately. the property we're discussing becomes useful and handy when you don't care about that distinction, which is quite often in practice.

for example a useful datatype is an association list. (setq x ((a . 1) (b . 2) (c . nil))) you can query it by calling (assoc 'a x) which is going to give you back a cons cell (a . 1) in this case. now the presence or absence of this cell indicates the association. if you want to know explicitly that C is nil, then you have an option to, and it's similar in function call counts to Scheme. if you don't care though about the distinction you can do (cdr (assoc 'a x)) which is going to give you 1. doing (cdr (assoc 'foo x)) will give you nil without erroring out. it's a pretty common pattern.

in case of established data types like association list, you will probably have a library of useful functions already defined, like you can write your own getassoc function that hides the above. you can also return multiple values from getassoc the same way as gethash does the first value being the value, and the second value being whether or not there's a corresponding cons cell.

but when you define your own adhoc cons cell based structures, you don't have the benefit of predefined functions. so let's say you have an association list of symbols to cons cells (setq x ((a . (foo . 1)) (b . (bar . 2)) (c . nil))). if I want to get foo out of that list, I'll say (cadr (assoc x 'a)) which will return foo. doing (cadr (assoc x 'c)) or (cadr (assoc x 'missing)) will both return nil. these later manipulations require extensive scaffolding in Scheme.

We can declare that our code only works with lists of numbers, or lists of strings. Therefore, nil is not expected. If (car list) returns nil, it can only be that the list is empty, because if it were not empty, it would return a number, or string, or widget or whatever the list holds.

Even when we have a heterogeneous list in Lisp, like one that can have symbols, numbers, strings or widgets, we can almost always exclude nil as a matter of design, and thus cheerfully use the simpler code.

We cannot exclude nil when a list contains Boolean values, because nil is our false.

We also cannot exclude it when it contains lists, because nil is our empty list.

The beauty is that in many situations, we can arrange not to have to care about the distinction between "item is missing" and "item is false" and "item is an empty list", and then we can write terser code.

When you see such terse code from another programmer, you know instinctively what the deal is with how they are treating nil before even looking at any documentation or test cases.

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