Now you can implement methods on this generic function that specialize on type with **EQL** specializers and return column objects with the slots filled in with appropriate values. Here’s the generic function and methods that define column types for the type names string and number:

    1. (defmethod make-column (name (type (eql 'string)) &optional default-value)
    2.   (make-instance
    3.    'column 
    4.    :name name
    5.    :comparator #'string< 
    6.    :equality-predicate #'string=
    7.    :default-value default-value
    8.    :value-normalizer #'not-nullable))
    11.    'column
    12.    :name name
    13.    :comparator #'< 
    14.    :equality-predicate #'=
    15.    :default-value default-value))

    The following function, not-nullable, used as the value-normalizer for string columns, simply returns the value it’s given unless the value is **NIL**, in which case it signals an error:

    Another column type you’ll need for the MP3 database is an interned-string whose values are interned as discussed previously. Since you need a hash table in which to intern values, you should define a subclass of column, interned-values-column, that adds a slot whose value is the hash table you use to intern.

    To implement the actual interning, you’ll also need to provide an :initform for value-normalizer of a function that interns the value in the column’s interned-values hash table. And because one of the main reasons to intern values is to allow you to use **EQL** as the equality predicate, you should also add an :initform for the equality-predicate of .

    1. (defclass interned-values-column (column)
    2.   ((interned-values
    3.     :reader interned-values
    4.    (equality-predicate :initform #'eql)
    5.    (value-normalizer   :initform #'intern-for-column)))
    7. (defun intern-for-column (value column)
    8.   (let ((hash (interned-values column)))
    9.     (or (gethash (not-nullable value column) hash)
    10.         (setf (gethash value hash) value))))

    With these methods defined on make-column, you can now define a function, make-schema, that builds a list of column objects from a list of column specifications consisting of a column name, a column type name, and, optionally, a default value.

    1. (defun make-schema (spec)
    2.   (mapcar #'(lambda (column-spec) (apply #'make-column column-spec)) spec))

    For instance, you can define the schema for the table you’ll use to store data extracted from MP3s like this: