RE: [External] : Executing function only if another function is defined

[carlmarcos]

Jul 24, 2022, 14:33 by drew.adams@oracle.com:

>> I would like to test whether the function `toucan'
>> has been defined before I call `docboost'.
>> What can I do?
>>
>
> Defined where?  By who/what?  What's the context?
>
> Predefined as part of Emacs?  Defined in some
> 3rd-party package?  Defined in some library that
> you've already loaded in the current session?
>
> Defined under the name `toucan'?  Defined under
> another name (or anonymously) but with the same
> code or similar behavior?
>

I define the function toucan in one of my init files using (require 'myfile)

Sometimes I comment out the require part, meaning that the function toucan 
would not be loaded.  Thus it does not make sense to a information to its 
docstring.



> A Lisp function, i.e., funcallable?  A "function"
> more generally (a "function-like object"), i.e.,
> a symbol with a non-nil `symbol-function' value?
>
>
>
> ___
>
> -- Function: functionp object
> This function returns ‘t’ if OBJECT is any kind of function, i.e.,
> can be passed to ‘funcall’.  Note that ‘functionp’ returns ‘t’ for
> symbols that are function names, and returns ‘nil’ for special
> forms.
>
> -- Function: fboundp symbol
> This function returns ‘t’ if the symbol has an object in its
> function cell, ‘nil’ otherwise.  It does not check that the object
> is a legitimate function.
>
> -- Function: symbol-function symbol
> This returns the object in the function cell of SYMBOL.  It does
> not check that the returned object is a legitimate function.
>
> If the function cell is void, the return value is ‘nil’.  To
> distinguish between a function cell that is void and one set to
> ‘nil’, use ‘fboundp’ (see below).
> ...
>
> If you have never given a symbol any function definition, we say that
> that symbol’s function cell is “void”.  In other words, the function
> cell does not have any Lisp object in it.  If you try to call the symbol
> as a function, Emacs signals a ‘void-function’ error.
>
> Note that void is not the same as ‘nil’ or the symbol ‘void’.  The
> symbols ‘nil’ and ‘void’ are Lisp objects, and can be stored into a
> function cell just as any other object can be (and they can be valid
> functions if you define them in turn with ‘defun’).  A void function
> cell contains no object whatsoever.
>
> You can test the voidness of a symbol’s function definition with
> ‘fboundp’.  After you have given a symbol a function definition, you can
> make it void once more using ‘fmakunbound’.
> ___
>
> FWIW -
>
> Your questions tend to be guessing games for
> anyone who might want to help.  (What is it that
> he's asking?  Is it bigger than a bread basket?
> Animal?  Vegetable?  Mineral?)
>
> Do _yourself_ a favor and _ask yourself_ first
> what it is that you really want to ask.
>
> Getting an answer starts with understanding
> your own question.  If you can't explicate your
> question then you don't really understand what
> you're asking, as well as you should.  Asking
> it (badly) at that point can waste your time as
> well as that of others.
> ___
>
> Elisp manual menu, node `Functions':
>
> 13 Functions
> ************
>
> A Lisp program is composed mainly of Lisp functions.  This chapter
> explains what functions are, how they accept arguments, and how to
> define them.
>
> * Menu:
>
> * What Is a Function::          Lisp functions vs. primitives; terminology.
> * Lambda Expressions::          How functions are expressed as Lisp objects.
> * Function Names::              A symbol can serve as the name of a function.
> * Defining Functions::          Lisp expressions for defining functions.
> * Calling Functions::           How to use an existing function.
> * Mapping Functions::           Applying a function to each element of a 
> list, etc.
> * Anonymous Functions::         Lambda expressions are functions with no 
> names.
> * Generic Functions::           Polymorphism, Emacs-style.
> * Function Cells::              Accessing or setting the function definition
>  of a symbol.
> * Closures::                    Functions that enclose a lexical environment.
> * Advising Functions::          Adding to the definition of a function.
> * Obsolete Functions::          Declaring functions obsolete.
> * Inline Functions::            Functions that the compiler will expand 
> inline.
> * Declare Form::                Adding additional information about a 
> function.
> * Declaring Functions::         Telling the compiler that a function is 
> defined.
> * Function Safety::             Determining whether a function is safe to 
> call.
> * Related Topics::              Cross-references to specific Lisp primitives
>  that have a special bearing on how functions work.
> ___
>
> Elisp manual `Index', entries starting with
> "function" (all with completion from `i'):
>
> * funcall:                               Calling Functions.   (line  22)
> * funcall <1>:                           Module Misc.         (line  56)
> * funcall, and debugging:                Internals of Debugger.
>  (line  86)
> * funcall-interactively:                 Interactive Call.    (line  73)
> * function:                              Anonymous Functions. (line  39)
> * function aliases:                      Defining Functions.  (line  53)
> * function call:                         Function Forms.      (line   6)
> * function call debugging:               Function Debugging.  (line   6)
> * function cell:                         Symbol Components.   (line  16)
> * function cell in autoload:             Autoload.            (line  70)
> * function declaration:                  Declaring Functions. (line   6)
> * function definition:                   Function Names.      (line   6)
> * function descriptions:                 A Sample Function Description.
>  (line   6)
> * function form evaluation:              Function Forms.      (line   6)
> * function input stream:                 Input Streams.       (line  26)
> * function invocation:                   Calling Functions.   (line   6)
> * function keys:                         Function Keys.       (line   6)
> * function name:                         Function Names.      (line   6)
> * function not known to be defined, compilation warning: Compiler Errors.
>  (line  21)
> * function output stream:                Output Streams.      (line  24)
> * function quoting:                      Anonymous Functions. (line  40)
> * function safety:                       Function Safety.     (line   6)
> * function-documentation property:       Documentation Basics.
>  (line  12)
> * function-get:                          Symbol Plists.       (line  58)
> * function-put:                          Symbol Plists.       (line  68)
> * functionals:                           Calling Functions.   (line 107)
> * functionp:                             What Is a Function.  (line  95)
> * functions in modes:                    Major Mode Conventions.
>  (line  55)
> * functions, making them interactive:    Defining Commands.   (line   6)
> ___
>
> Someone has gone to a lot of trouble to lay
> out for you the answers to your questions.
> You just need to learn how to _Ask Emacs_.
> And then try your best to do that.
>