[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
dynamic and lexical scope, attempted summary with example (was: Re: run-
From: |
Emanuel Berg |
Subject: |
dynamic and lexical scope, attempted summary with example (was: Re: run-with-timer does not display message) |
Date: |
Sun, 20 Jul 2014 23:28:58 +0200 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/24.3 (gnu/linux) |
As always, I think it is easier to discuss in terms of
an example...
This function is what started it all:
(defun shortcut-to-file (key-prefix key file-prefix file)
"Make shortcut with key KEY-PREFIX KEY to FILE-PREFIX FILE."
(global-set-key
(format "%s%s" key-prefix key)
`(lambda ()
(interactive)
(do-show-file (format "%s%s" ,file-prefix ,file)) )))
Is this correct?
In dynamic scope:
1. At the 4th line, the line that starts with `format',
we see use of the arguments to the parameters
`key-prefix' and `key' - because they are inside the
defun, it is clear that those refer the the
arguments passed as parameters.
2. At the 7th, last line, `file-prefix' and `file'
appears. Those are preceded by commas so, in
combination with the backtick, their values will be
inserted - hard-coded. This bypasses the whole
problem because it eliminates all evaluation of
symbols (it is like Caesar and the Gordian
knot). The drawback is that the lambda can't be
byte-compiled as it is formally "just" a list.
3. If the lambda wasn't backticked and shot with
commas, the defun would evaluate fine and even
calling it wouldn't raise an error. However, when
the keystroke was actually hit, the lambda wouldn't
be within the defun anymore - it would be in the
wild, and `file-prefix' and `file' would be
interpreted as symbols, i.e., global
variables. Those are probably not defined... but if
they were, with `setq' for example, those values
would be used (this would make for a total mess).
In lexical scope, the backtick and the commas aren't
necessary. The lambda can be expressed as a lambda and
nothing else, which means it can be byte-compiled. When
the symbols are encountered, those aren't looked for as
global variables, but rather the history of the place
they occurred in code is drilled down, until a scope is
encountered where they are defined, and those values
are used.
--
underground experts united
- Re: run-with-timer does not display message, (continued)
- Re: run-with-timer does not display message, Sebastian Wiesner, 2014/07/20
- Re: run-with-timer does not display message, Stefan Monnier, 2014/07/21
- Message not available
- Re: run-with-timer does not display message, Emanuel Berg, 2014/07/20
- Re: run-with-timer does not display message, Stefan Monnier, 2014/07/21
- Message not available
- Re: run-with-timer does not display message, Emanuel Berg, 2014/07/21
- Re: run-with-timer does not display message, Stefan Monnier, 2014/07/24
- Message not available
- Re: run-with-timer does not display message, Emanuel Berg, 2014/07/20
- RE: run-with-timer does not display message, Drew Adams, 2014/07/20
- Message not available
- dynamic and lexical scope, attempted summary with example (was: Re: run-with-timer does not display message),
Emanuel Berg <=
- Re: dynamic and lexical scope, attempted summary with example, Thien-Thi Nguyen, 2014/07/21
- Re: dynamic and lexical scope, attempted summary with example, Thorsten Jolitz, 2014/07/21
- Message not available
- Re: dynamic and lexical scope, attempted summary with example, Emanuel Berg, 2014/07/21
- Re: dynamic and lexical scope, attempted summary with example, Michael Heerdegen, 2014/07/21
- Message not available
- Re: dynamic and lexical scope, attempted summary with example, Emanuel Berg, 2014/07/21
- Re: dynamic and lexical scope, attempted summary with example, Thien-Thi Nguyen, 2014/07/21
- Message not available
- Re: dynamic and lexical scope, attempted summary with example, Emanuel Berg, 2014/07/21
- Message not available
- Re: dynamic and lexical scope, attempted summary with example, Emanuel Berg, 2014/07/21
- Message not available
- Re: run-with-timer does not display message, Emanuel Berg, 2014/07/19
Message not available