bug#70155: 29.3; Several Emacs Lisp list functions accept non-list arguments

[Mattias Engdegård]

27 apr. 2024 kl. 10.22 skrev Eli Zaretskii <eliz@gnu.org>:

> Mattias, Stefan: any comments on this?  Should we document this, or
> should we change the code?

Eli, thank you for bringing this to our attention. I personally don't see much 
that we need to change.
Our priorities in Lisp are, arguably:

1. Well-defined behaviour should work as documented.
2. Undefined behaviour (ie, usage errors) should not cause crashes or 
corruption. This is what we usually call 'safety', or protecting the system's 
own abstractions.
3. Useful error conditions (such as file-not-found) should work as documented 
or at least be handled in a predictable and useful way.
4. The implementation should be efficient.
5. Incorrect usage should be detected and signalled as far as pragmatically 
possible, to help programmers find mistakes in their code.

The low ranking of the last item means that we don't necessarily promise to 
signal an error for every mistake a programmer can make.

However, in this case there are also algebraic motivations for the behaviour:

> From: <tpeplt@gmail.com>
> Date: Tue, 02 Apr 2024 19:15:28 -0400
> 
>> The built-in Emacs Lisp functions ‘last’, ‘nthcdr’, ‘take’,
>> and ‘ntake’ are functions that accept lists as an argument.
>> However, they also accept non-list arguments without
>> signaling an error.

(nth 1 '(a b . c)) does not signal an error nor do we expect it to, so it's 
reasonable that

  (take 2 '(a b . c)) -> (a b)
  (take 1 '(a . b))   -> (a)
  (take 0 'a)         -> ()

do not signal errors either. nthcdr is the complement of [n]take and behaves 
the same way with respect to nonpositive arguments, and in fact has the 
definition

  (nthcdr N L) = (cdr^N L)

for all naturals N, which means that

  (nthcdr 0 'a)       -> a

is correct.

(Of course if you ask me, I'd prefer it if lists were guaranteed to be proper, 
immutable, with the empty list an object distinct from the symbol nil and the 
false boolean value. Maybe next year.)

`last` works in the same way:

  (last '(xN ... x1 . x0) M)
  -> (last '(x{min(N,M)} ... x1 . x0) M)

with the base case

  (last '(xN ... x1 . x0) 0) -> x0

ie,

  (last x0 0) -> x0

for any atom x0.
Finally, [n]butlast are just [n]take with a different numeric argument, but 
here the special case

  (butlast X 0) -> X

is motivated by performance considerations; it's more important to have it go 
in constant time.