symbols.md

Symbols

Common Lisp is a Lisp 2. The only difference between a Lisp 1 and Lisp 2 is that Lisp 2 languages have two separate namespaces for values and functions. In Common Lisp this is managed using symbols. We can demonstrate this as follows, in the REPL:

CL-USER> (inspect 42)

The object is an ATOM:
  42

inspect is a function which reports on the internal representation of the object that is passed to it. In the example above, the number 42 which is an ATOM.

CL-USER> (inspect 'foo)

The object is a SYMBOL.
0. Name: "FOO"
1. Package: #<PACKAGE "COMMON-LISP-USER">
2. Value: "unbound"
3. Function: "unbound"
4. Plist: NIL

Here we inspect a symbol foo and we can see 5 "slots".

• Name which is a unique identifier for the symbol.
• Package which as you might expect defines a namespace in which the symbol exists.
• Value
• Function
• Plist is out of scope for this chapter.

We will look more deeply at Value and Function.

When we define a value "foo" we fill the Value slot on the symbol foo. For example:

CL-USER> (defparameter foo "some value")

FOO

CL-USER> (inspect 'foo)

The object is a SYMBOL.
0. Name: "FOO"
1. Package: #<PACKAGE "COMMON-LISP-USER">
2. Value: "some value"
3. Function: "unbound"
4. Plist: NIL

The Value slot of foo now contains "some value". Similarly, declaring a function foo will fill the Function slot on foo

CL-USER> (inspect '(a b c))

The object is a proper list of length 3.
0. 0: A
1. 1: B
2. 2: C

In this example, a list is passed to inspect. We can see that the list has a numbered slot for each item, each item has an index and also a value.