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

[Drew Adams]

> 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?

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.

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