What is the difference between a variable and a symbol in LISP?

Question

In terms of scope? Actual implementation in memory? The syntax? For eg, if (let a 1) Is \'a\' a variable or a symbol?

Answer 1

A symbol is a name for a thing. A variable is a mutable pointer to a mutable storage location.

In the code snippet you showed, both let and a are symbols. Within the scope of the let block, the symbol a denotes a variable which is currently bound to the value 1.

But the name of the thing is not the thing itself. The symbol a is not a variable. It is a name for a variable. But only in this specific context. In a different context, the name a can refer to a completely different thing.

Example: the symbol jaguar may, depending on context, denote

• OSX 10.2
• a gaming console
• a car manufacturer
• a ground attack military jet airplane
• another military jet airplane
• a supercomputer
• an electric guitar
• and a whole lot of other things
• oh, did I forget something?