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ftype

ftype Declaration

Syntax:

(ftype type {function-name}*)

Arguments:

function-name—a function name.

type—a type specifier .

Valid Context:

declaration or proclamation

Binding Types Aected:

function

Description:

Specifies that the functions named by function-names are of the functional type type. For example:

(declare (ftype (function (integer list) t) ith)

(ftype (function (number) float) sine cosine))

If one of the functions mentioned has a lexically apparent local definition (as made by flet or labels), then the declaration applies to that local definition and not to the global function definition. ftype declarations never apply to variable bindings (see type).

The lexically apparent bindings of function-names must not be macro definitions. (This is because ftype declares the functional definition of each function name to be of a particular subtype of function, and macros do not denote functions.)

ftype declarations can be free declarations or bound declarations. ftype declarations of functions that appear before the body of a flet or labels form that defines that function are bound declarations. Such declarations in other contexts are free declarations.

See Also:

declare, declaim, proclaim

Expanded Reference: ftype​

Declaring Function Types​

The ftype declaration specifies the function type signature of named functions -- their argument types and return type.

(declaim (ftype (function (integer integer) integer) my-add))

(defun my-add (a b)
  (+ a b))
=> MY-ADD

Local ftype Declarations​

(defun compute (x y)
  (declare (ftype (function (number number) number) my-add))
  (my-add x y))
=> COMPUTE

Declaring Multiple Functions​

(declaim (ftype (function (string) string) upcase-string)
         (ftype (function (list) integer) list-length))

Function Type with Optional Arguments​

(declaim (ftype (function (integer &optional integer) integer) add-opt))

(defun add-opt (a &optional (b 0))
  (+ a b))
=> ADD-OPT