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`read--expression' and `read-minibuffer'

From: Drew Adams
Subject: `read--expression' and `read-minibuffer'
Date: Sat, 3 Sep 2016 11:20:52 -0700 (PDT)

Function `read--expression' was added in Emacs 24.4.

Can someone please explain why this is an "internal" function
(two hyphens in the name)?  Why wouldn't users want to use
this to read a Lisp expression from the minibuffer?

It seems like `read--expression' should now be the usual way
to read and return a Lisp expression, and that it should NOT
be considered "internal".  No?

On the other hand, we still also have the longstanding
(non-internal) function `read-minibuffer' for reading and
returning a Lisp "object".

It's not clear to me how a Lisp object differs from a Lisp
sexp.  (And the doc of `interactive' even calls what "x"
reads a "Lisp expression", not a "Lisp object".)

Well, sure, there are Lisp objects (e.g. byte-compiled
functions) that are not readable expressions.  But those are
not readable anyway, AFAIK.  `read-minibuffer', like
`read--expression', just reads expressions, AFAICT.

The difference between `read-minibuffer' and `read--expression'
is this: The former uses `minibuffer-local-map', so it has no
special handling of Lisp expressions and no completion.  The
latter uses `read-expression-map', and it provides Lisp-sexp

Why isn't `read--expression' treated as a user function, like
`read-minibuffer', instead of being considered "internal"?

And `read-minibuffer' is still used for `interactive' with
spec "x".  Why isn't `read--expression' used for that now?

How about promoting `read--expression' to `read-expression'
and using it for `(interactive "x")'?  Nothing wrong with
also keeping `read-minibuffer' (though it's name is not great),
but why continue to use it in preference to `read--expression'?

`read-minibuffer' is still used in several places, but I
don't see why those places wouldn't benefit from using
`read--expression' instead.

Am I missing something?  I ask because I read Lisp sexps
from the minibuffer, and I don't (yet) see any reason to
use `read-minibuffer' instead of "internal" function
`read--expression'.  I'd like to learn which to use and why.

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