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(cl)Type Predicates

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Type Predicates
===============

The "CL" package defines a version of the Common Lisp `typep' predicate.

 - Function: typep object type
     Check if OBJECT is of type TYPE, where TYPE is a (quoted) type
     name of the sort used by Common Lisp.  For example, `(typep foo
     'integer)' is equivalent to `(integerp foo)'.

   The TYPE argument to the above function is either a symbol or a list
beginning with a symbol.

   * If the type name is a symbol, Emacs appends `-p' to the symbol
     name to form the name of a predicate function for testing the
     type.  (Built-in predicates whose names end in `p' rather than
     `-p' are used when appropriate.)

   * The type symbol `t' stands for the union of all types.  `(typep
     OBJECT t)' is always true.  Likewise, the type symbol `nil' stands
     for nothing at all, and `(typep OBJECT nil)' is always false.

   * The type symbol `null' represents the symbol `nil'.  Thus `(typep
     OBJECT 'null)' is equivalent to `(null OBJECT)'.

   * The type symbol `real' is a synonym for `number', and `fixnum' is
     a synonym for `integer'.

   * The type symbols `character' and `string-char' match integers in
     the range from 0 to 255.

   * The type symbol `float' uses the `floatp-safe' predicate defined
     by this package rather than `floatp', so it will work correctly
     even in Emacs versions without floating-point support.

   * The type list `(integer LOW HIGH)' represents all integers between
     LOW and HIGH, inclusive.  Either bound may be a list of a single
     integer to specify an exclusive limit, or a `*' to specify no
     limit.  The type `(integer * *)' is thus equivalent to `integer'.

   * Likewise, lists beginning with `float', `real', or `number'
     represent numbers of that type falling in a particular range.

   * Lists beginning with `and', `or', and `not' form combinations of
     types.  For example, `(or integer (float 0 *))' represents all
     objects that are integers or non-negative floats.

   * Lists beginning with `member' or `member*' represent objects `eql'
     to any of the following values.  For example, `(member 1 2 3 4)'
     is equivalent to `(integer 1 4)', and `(member nil)' is equivalent
     to `null'.

   * Lists of the form `(satisfies PREDICATE)' represent all objects
     for which PREDICATE returns true when called with that object as
     an argument.

   The following function and macro (not technically predicates) are
related to `typep'.

 - Function: coerce object type
     This function attempts to convert OBJECT to the specified TYPE.
     If OBJECT is already of that type as determined by `typep', it is
     simply returned.  Otherwise, certain types of conversions will be
     made:  If TYPE is any sequence type (`string', `list', etc.) then
     OBJECT will be converted to that type if possible.  If TYPE is
     `character', then strings of length one and symbols with
     one-character names can be coerced.  If TYPE is `float', then
     integers can be coerced in versions of Emacs that support floats.
     In all other circumstances, `coerce' signals an error.

 - Special Form: deftype name arglist forms...
     This macro defines a new type called NAME.  It is similar to
     `defmacro' in many ways; when NAME is encountered as a type name,
     the body FORMS are evaluated and should return a type specifier
     that is equivalent to the type.  The ARGLIST is a Common Lisp
     argument list of the sort accepted by `defmacro*'.  The type
     specifier `(NAME ARGS...)' is expanded by calling the expander
     with those arguments; the type symbol `NAME' is expanded by
     calling the expander with no arguments.  The ARGLIST is processed
     the same as for `defmacro*' except that optional arguments without
     explicit defaults use `*' instead of `nil' as the "default"
     default.  Some examples:

          (deftype null () '(satisfies null))    ; predefined
          (deftype list () '(or null cons))      ; predefined
          (deftype unsigned-byte (&optional bits)
            (list 'integer 0 (if (eq bits '*) bits (1- (lsh 1 bits)))))
          (unsigned-byte 8)  ==  (integer 0 255)
          (unsigned-byte)  ==  (integer 0 *)
          unsigned-byte  ==  (integer 0 *)

     The last example shows how the Common Lisp `unsigned-byte' type
     specifier could be implemented if desired; this package does not
     implement `unsigned-byte' by default.

   The `typecase' and `check-type' macros also use type names.  Note:
Conditionals.  Note: Assertions.  The `map', `concatenate', and
`merge' functions take type-name arguments to specify the type of
sequence to return.  Note: Sequences.