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4. Predicates

This section describes functions for testing whether various facts are true or false.

4.1 Type Predicates  `typep', `deftype', and `coerce'
4.2 Equality Predicates  `eql' and `equalp'


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4.1 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.

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.

Macro: 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. See section 5.4 Conditionals. See section 14. Assertions and Errors. The map, concatenate, and merge functions take type-name arguments to specify the type of sequence to return. See section 10. Sequences.


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4.2 Equality Predicates

This package defines two Common Lisp predicates, eql and equalp.

Function: eql a b
This function is almost the same as eq, except that if a and b are numbers of the same type, it compares them for numeric equality (as if by equal instead of eq). This makes a difference only for versions of Emacs that are compiled with floating-point support, such as Emacs 19. Emacs floats are allocated objects just like cons cells, which means that (eq 3.0 3.0) will not necessarily be true--if the two 3.0s were allocated separately, the pointers will be different even though the numbers are the same. But (eql 3.0 3.0) will always be true.

The types of the arguments must match, so (eql 3 3.0) is still false.

Note that Emacs integers are "direct" rather than allocated, which basically means (eq 3 3) will always be true. Thus eq and eql behave differently only if floating-point numbers are involved, and are indistinguishable on Emacs versions that don't support floats.

There is a slight inconsistency with Common Lisp in the treatment of positive and negative zeros. Some machines, notably those with IEEE standard arithmetic, represent +0 and -0 as distinct values. Normally this doesn't matter because the standard specifies that (= 0.0 -0.0) should always be true, and this is indeed what Emacs Lisp and Common Lisp do. But the Common Lisp standard states that (eql 0.0 -0.0) and (equal 0.0 -0.0) should be false on IEEE-like machines; Emacs Lisp does not do this, and in fact the only known way to distinguish between the two zeros in Emacs Lisp is to format them and check for a minus sign.

Function: equalp a b
This function is a more flexible version of equal. In particular, it compares strings and characters case-insensitively, and it compares numbers without regard to type (so that (equalp 3 3.0) is true). Vectors and conses are compared recursively. All other objects are compared as if by equal.

This function differs from Common Lisp equalp in several respects. In keeping with the idea that strings are less vector-like in Emacs Lisp, this package's equalp also will not compare strings against vectors of integers.

Also note that the Common Lisp functions member and assoc use eql to compare elements, whereas Emacs Lisp follows the MacLisp tradition and uses equal for these two functions. In Emacs, use member* and assoc* to get functions which use eql for comparisons.


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