Dynamic Type Checking

Dynamic type checking, used in Lisp, checks types at runtime before operations are performed on data.

• Types are associated with values. Requires the ability to determine the type of any data at runtime, e.g. by storing a type tag with data. Or, pages of memory can be allocated to specific types; the type of data can be looked up in a table indexed by page address.

• Slower at runtime due to repeated type checking.

• More flexible: union types can be supported, as in the copy-tree function.

  
  (defun copy-tree (x)
    (if (consp x)
        (cons (copy-tree (first x))
              (copy-tree (rest x)))
        x))
  

Common Lisp combines dynamic type checking with optional type declarations to allow compilation of code using static checks.