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Re: Quoted function in `define-key'


From: Michael Heerdegen
Subject: Re: Quoted function in `define-key'
Date: Sat, 04 Feb 2017 18:25:40 +0100
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/26.0.50 (gnu/linux)

Narendra Joshi <address@hidden> writes:

>    (quote my-function) is exactly the same as 'my-function, i.e.

Yes, the second is an alternative read syntax for the first expression.

>    (eq (quote my-function) 'my-function) evaluates to t.
>
>    But here we are talking about the result of evaluating
>
>    (quote my-function) which isn't a cons but a symbol.

No, it's a list (and a cons), but the result of evaluation is a symbol.

>     But if we have `(quote my-function) that isn't the same as
>    'my-function.

`(quote my-function) and '(quote my-function) eval to the list
(quote my-function).  ''my-function is an alternative read syntax for
'(quote my-function).  But that all doesn't appear in the example.

It's so: you want to specify a symbol as third argument to `define-key'.
`define-key' is a function, so the argument positions are evaluated.
Thus you want to specify an expression that evaluates to the symbol you
want.  (quote my-function) or 'my-function evaluate to
the symbol you want.  In

   (define-key rinari-prefix-map (car el) (cdr el))

the expression at that position is (cdr el), so that expression should
eval to a symbol, not to a list like (quote symbol) - even when this
list would give you what you want when it would be evaluated.
Evaluation happens only once.

With other words: when you write something like

  (define-key my-map keys 'my-function)

you use the quote to prevent my-function from being evaluated, because
you want to specify that symbol (unlike its binding as a variable), but
_not_ because define-key would expect something quoted.

BTW, most people prefer

  (define-key my-map keys #'my-function)

however.  #'my-function is a read syntax for (function my-function),
where function is like quote but tells the byte compiler that the quoted
thing is a function expression.

>    Side note: If anybody knows how to type inline code in gnus message,
>    it would be greatly appreciated.

I use `message-mark-inserted-region' for multiline code, but I don't
know of any rule for smaller snippets.  Most of the time I don't care
too much (like here).  It's good style to quote a `symbol' like this,
but I sometimes avoid it when code snippets already involve lots of
quoting.


HTH,

Michael.



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