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4 points by kennytilton 4248 days ago | link | parent

<cough> No, the /code/ you offered was looking for a list with less than a given length with the understanding that nils might be in the list. The test for a list without nils is:

   (all idfn lst)
Is it a kluge to state the test positively? Actually, negative tests are understood to be harder on the cognitive system to get right.

Meanwhile, are you sure (member nil lst) is a kluge? It sure looks readable, and it sure does work. What exactly bothers you about that?

Meanwhile, a use case means your are not just running code samples you typed in at the repl, it means you are trying to solve a problem using a computer and something came up. So here we are after iterating over the members of the library and we built up a list of overdue books per member -- no, that would not have nils, that would have an assoc entry (bob . nil). Hmmm... can you help me here, I cannot think of one.

You have not responded to the very similar example of tables. Correct, one cannot distinguish the absence of a key from a key added with the value nil. A use case is not a couple of lines of code proving that, a use case is a lookup on drivers license number during a traffic stop and...?

My point you may have missed is that without a use case not perfectly well expressed with either of the two solutions I offered above, the fundamental weakness is in offering a theoretical functional gap in a language as if it were a gap in a language.

Recall that pg is already on record as wanting things that are problems in practice, that this is a hacker's language, not a strait jacket language like C++ or Java with all sorts of theoretical angst over untyped variables.

The problem, btw, of putting an air bag in the trunk of a car is the added weight. :)

1 point by byronsalty 4247 days ago | link

In practice I imagine one would just use a known symbol like 'removed mid list/tree instead of blindly using a nil. But this is much less fun.


2 points by kennytilton 4245 days ago | link

Puzzled. Do you mean the nils would arise because non-nil entries would be changed to nil as they were (somehow) processed? Just delete the cons:

   (= (cdr prior-kill-me) (cdr kill-me))
instead of:

   (= (car kill-me) nil)
Otherwise you are forever iterating over the now irrelevant cons,