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From: Jesper Harder <harder@myrealbox.com>
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Subject: Re: What does the coding system nil mean?
Date: Mon, 26 Jan 2004 16:20:22 +0100
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Eli Zaretskii <eliz@elta.co.il> writes:

>> Then nil isn't really a coding system, but just a value that some
>> coding system related functions happen to interpret in a certain way.
>
> It's a coding system in the sense that every primitive that accepts
> a coding system symbol also accepts nil.

But it's not transparent, i.e. some functions behave differently if
you use nil rather than a real coding system.  For example

    (progn
      (setq last-coding-system-used 'none-such)
      (encode-coding-string "foo" nil)
      last-coding-system-used)

    => none-such

If nil was a real coding system, the value of
`last-coding-system-used' would have been changed.  There are also
other cases, where you can tell that nil is not a coding system.

>> Though, I think it's a bit odd for a predicate called
>> `coding-system-p' to return t for an object that is _not_ in fact a
>> coding system.
>
> IMHO, it's no more odd than this:
>
>    M-: (listp nil) RET => t

I would find it a lot more odd if (listp ()) did not return t -- nil
is the empty list, after all.