The question is not about using keywords, but actually about keyword implementation. For example, when I create some function with keyword parameters and make a call:
(defun fun (&key key-param) (print key-param)) => FUN
(find-symbol "KEY-PARAM" 'keyword) => NIL, NIL ;;keyword is not still registered
(fun :key-param 1) => 1
(find-symbol "KEY-PARAM" 'keyword) => :KEY-PARAM, :EXTERNAL
How a keyword is used to pass an argument? Keywords are symbols whos values are themselves, so how a corresponding parameter can be bound using a keyword?
Another question about keywords — keywords are used to define packages. We can define a package named with already existing keyword:
(defpackage :KEY-PARAM) => #<The KEY-PARAMETER package, 0/16 ...
(in-package :KEY-PARAM) => #<The KEY-PARAMETER package, 0/16 ...
(defun fun (&key key-param) (print key-param)) => FUN
(fun :KEY-PARAM 1) => 1
How does system distinguish the usage of :KEY-PARAM between package name and function parameter name?
Also we can make something more complicated, if we define function KEY-PARAM and export it (actually not function, but name):
(in-package :KEY-PARAM)
(defun KEY-PARAM (&key KEY-PARAM) KEY-PARAM) => KEY-PARAM
(defpackage :KEY-PARAM (:export :KEY-PARAM))
;;exporting function KEY-PARAM, :KEY-PARAM keyword is used for it
(in-package :CL-USER) => #<The COMMON-LISP-USER package, ...
(KEY-PARAM:KEY-PARAM :KEY-PARAM 1) => 1
;;calling a function KEY-PARAM from :KEY-PARAM package with :KEY-PARAM parameter...
The question is the same, how Common Lisp does distinguish the usage of keyword :KEY-PARAM here?
If there is some manual about keywords in Common Lisp with explanation of their mechanics, I would be grateful if you posted a link here, because I could find just some short articles only about usage of the keywords.
See Common Lisp Hyperspec for full details of keyword parameters. Notice that
(defun fun (&key key-param) ...)
is actually short for:
(defun fun (&key ((:key-param key-param)) ) ...)
The full syntax of a keyword parameter is:
((keyword-name var) default-value supplied-p-var)
default-value and supplied-p-var are optional. While it's conventional to use a keyword symbol as the keyword-name, it's not required; if you just specify a var instead of (keyword-name var), it defaults keyword-name to being a symbol in the keyword package with the same name as var.
So, for example, you could do:
(defun fun2 (&key ((myoption var))) (print var))
and then call it as:
(fun 'myoption 3)
The way it works internally is when the function is being called, it steps through the argument list, collecting pairs of arguments <label, value>. For each label, it looks in the parameter list for a parameter with that keyword-name, and binds the corresponding var to value.
The reason we normally use keywords is because the : prefix stands out. And these variables have been made self-evaluating so that we don't have to quote them as well, i.e. you can write :key-param instead of ':key-param (FYI, this latter notation was necessary in earlier Lisp systems, but the CL designers decided it was ugly and redundant). And we don't normally use the ability to specify a keyword with a different name from the variable because it would be confusing. It was done this way for full generality. Also, allowing regular symbols in place of keywords is useful for facilities like CLOS, where argument lists get merged and you want to avoid conflicts -- if you're extending a generic function you can add parameters whose keywords are in your own package and there won't be collisions.
The use of keyword arguments when defining packages and exporting variables is again just a convention. DEFPACKAGE, IN-PACKAGE, EXPORT, etc. only care about the names they're given, not what package it's in. You could write
(defpackage key-param)
and it would usually work just as well. The reason many programmers don't do this is because it interns a symbol in their own package, and this can sometimes cause package conflicts if this happens to have the same name as a symbol they're trying to import from another package. Using keywords divorces these parameters from the application's package, avoiding potential problems like this.
The bottom line is: when you're using a symbol and you only care about its name, not its identity, it's often safest to use a keyword.
Finally, about distinguishing keywords when they're used in different ways. A keyword is just a symbol. If it's used in a place where the function or macro just expects an ordinary parameter, the value of that parameter will be the symbol. If you're calling a function that has &key arguments, that's the only time they're used as labels to associate arguments with parameters.