From: Tom Capey <tom.capey@gmail.com>
To: Xue Fuqiao <xfq.free@gmail.com>
Cc: help-gnu-emacs@gnu.org
Subject: Re: About an example in Emacs Lisp manual
Date: Sun, 10 Feb 2013 15:07:29 +0000 [thread overview]
Message-ID: <87zjzcmclq.fsf@madasafish.co.uk> (raw)
In-Reply-To: <mailman.19496.1360505980.855.help-gnu-emacs@gnu.org> (Xue Fuqiao's message of "Sun, 10 Feb 2013 22:19:25 +0800")
* Xue Fuqiao writes:
> In (info "(elisp) Translation Keymaps"):
> For example, here's how to define `C-c h' to turn the character that
> follows into a Hyper character:
> (defun hyperify (prompt)
> (let ((e (read-event)))
> (vector (if (numberp e)
> (logior (lsh 1 24) e)
> (if (memq 'hyper (event-modifiers e))
> e
> (add-event-modifier "H-" e))))))
[...]
> In the first `defun' form, if `e' is a number,
> (vector (logior (lsh 1 24) e)) will be returned. I don't
> understand what the meaning of the bitwise-or and bit-shifting
> functions are here.
The Hyper modifier bit is the 2^24 bit--(info "(elisp) Other Char Bits").
If that bit is a 1 then it's on, 0 and it's off.
(lsh 1 n) returns a value with just the nth bit turned on, and
all the other bits are turned off, that is, are zero. Thus
(lsh 1 24) has the 2^24 bit turned on [(= (expt 2 24) (lsh 1 24)) => t],
the Hyper modifier bit.
`logior' does a bit-by-bit OR comparision between the number
with its 24th bit on and `e', looking at both numbers as though
they were binary representations. In this case we're OR-ing
`e' against the Hyper bit, and this has the effect of returning
`e' with the Hyper bit turned on.
/Tom
--
next parent reply other threads:[~2013-02-10 15:07 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <mailman.19496.1360505980.855.help-gnu-emacs@gnu.org>
2013-02-10 15:07 ` Tom Capey [this message]
2013-02-10 15:31 ` About an example in Emacs Lisp manual Xue Fuqiao
2013-02-10 14:19 Xue Fuqiao
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