Data Types

The way that data values are represented in Lisp is fundamentally different to more "conventional" languages such as C or Pascal: in Lisp each piece of data (a Lisp Object) has two basic attributes, the actual data and a tag value defining the type of the object. This means that type checking is performed on the actual data itself, not on the "variable" holding the data.

All Lisp objects are a member of one of the primitive types; these are types built into the Lisp system and can represent things like strings, integers, cons cells, vectors, etc...

More complex types of object can be constructed from these primitive types, for example a vector of three elements could be regarded as a type triple if necessary. In general, each separate type provides a predicate function which returns t when applied to an object of its type.

• Types Summary: List of the most common types
• Read Syntax: Some types can be constructed from source code
• Printed Representation: All types can be printed
• Equality Predicates: How to test two objects for equality
• Comparison Predicates: Comparing two objects as scalars
• Type Predicates: Each type has a predicate defining it
• Garbage Collection: Reusing memory from stale objects

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