Metaprogramming

Metaprogramming

[Shyam Nath]

🖖 Hello World,



This is a  query from over half a decade ago: https://stackoverflow.com/q/22224045



I want to redefine functions by manipulating the pre-existing function; is there any way to do this?



I'd rather not just copy the whole function definition into my init.el file just to change one LOC; maybe there's a better way of doing this part instead?



LIVE LONG AND PROSPER \V/,
AGENT S.
🕴
Migrating Infestive Bugs !!

Re: Metaprogramming

[Teemu Likonen]

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Shyam Nath [2019-09-15T13:13:09+10] wrote:

> I want to redefine functions by manipulating the pre-existing
> function; is there any way to do this?

You can keep the original function code in a variable. Use that variable
to define the global function first and later all modified versions of
it.


    (defvar function-code (lambda (...) ...))
    
    (setf (symbol-function 'modifiable-function) function-code)


I think it is better to keep your defuns static and make them call
other functions which are stored in a variable or other predefined
place.


    (defvar modifiable-function (lambda (...) ...))

    (defun static-function (...)
      ;; ...
      (funcall modifiable-function ...)
      ;; ...
      )

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Re: Metaprogramming

[Shyam Nath]

🖖 Hello Teemu,



I am doing the 2ⁿᵈ option: https://stackoverflow.com/a/15725437



How can I access functions before they load?



LIVE LONG AND PROSPER \V/,
AGENT S.
🕴
Migrating Infestive Bugs !!



---- On Sun, 15 Sep 2019 17:32:59 +1000 Teemu Likonen <tlikonen@iki.fi> wrote ----


Shyam Nath [2019-09-15T13:13:09+10] wrote: 
 
> I want to redefine functions by manipulating the pre-existing 
> function; is there any way to do this? 
 
You can keep the original function code in a variable. Use that variable 
to define the global function first and later all modified versions of 
it. 
 
 
 (defvar function-code (lambda (...) ...)) 
 
 (setf (symbol-function 'modifiable-function) function-code) 
 
 
I think it is better to keep your defuns static and make them call 
other functions which are stored in a variable or other predefined 
place. 
 
 
 (defvar modifiable-function (lambda (...) ...)) 
 
 (defun static-function (...) 
 ;; ... 
 (funcall modifiable-function ...) 
 ;; ... 
 ) 
 
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/ https://keybase.io/tlikonen https://github.com/tlikonen

Re: Metaprogramming

[Stefan Monnier]

> This is a  query from over half a decade ago: https://stackoverflow.com/q/22224045

That's not exactly about meta-programming, but rather about a peliminary
step you may (or may not) need for some meta-programming tasks.

> I want to redefine functions by manipulating the pre-existing
> function; is there any way to do this?

Yes, of course.  For example `advice-add` lets you do that.

> I'd rather not just copy the whole function definition into my init.el file
> just to change one LOC;
> maybe there's a better way of doing this part instead?

Then it depends on the specifics:
- what line of code?
- is the function usually compiled?
- is the source code available?
- is there a way to get that result differently?


        Stefan

Re: Metaprogramming

[Stefan Monnier]

> Then it depends on the specifics:
> - what line of code?
> - is the function usually compiled?
> - is the source code available?
> - is there a way to get that result differently?

BTW, you may want to look at `el-patch`.


        Stefan

Re: Metaprogramming

[Emanuel Berg via Users list for the GNU Emacs text editor]

Stefan Monnier wrote:

>> This is a query from over half a decade ago:
>> https://stackoverflow.com/q/22224045
>
> That's not exactly about meta-programming [...]

"Metaprogramming is a programming technique in
 which computer programs have the ability to
 treat other programs as their data. It means
 that a program can be designed to read,
 generate, analyze or transform other programs,
 and even modify itself while running." [1]

One should probably have a good, or at least
explicit reason to do any of that, since
otherwise it'll just make things complicated
where they don't have to be. However if one
finds such a use case and pulls it off, that's
programming at the next level, no doubt.


[1] https://en.wikipedia.org/wiki/Metaprogramming

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