defstar: macros allowing easy inline type declarations for variables and and function return values.

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Paul Sexton <>


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The DEFSTAR library

Defstar is a collection of Common Lisp macros that can be used in place of defun, defmethod, defgeneric, defvar, defparameter, flet, labels, let* and lambda. Each macro has the same name as the form it replaces, with a star added at the end, e.g. defun*. (the exception is the let* replacement, which is called *let).

The macros allow:

  • easy inclusion of type declarations for arguments in lambda lists
  • easy inclusion of return type declarations in function and method definitions
  • easy declaration of variables as 'ignored', by the use of '_' as a placeholder variable in argument lists.
  • easy inclusion of assertions for each argument and for the function's return value, thus allowing simple programming by contract.

See below for a detailed discussion of each macro, with usage examples.

Defstar's home is at

Installation requires ASDF or Quicklisp. Defstar does not depend on any other libraries.

Function definitions

The function-defining macros (defun*, flet*, labels*, lambda*) work the same as their normal counterparts, with the following exceptions:

  • Where the name of an argument appears in a lambda list, a clause of the form (name TYPE [TEST]) may appear instead. TYPE is any legal Common Lisp type declaration, for example integer or (real 0 100). TEST is an optional test which the supplied value of the argument must pass at runtime. This can be either an expression, or a symbol which will be taken to be the name of a predicate function which will be called with the value of name as its sole argument.
  • Where the name of the function appears, a clause of the form (fname -> TYPE [TEST]) may appear instead. Here, TYPE can be any legal Common Lisp type declaration, and refers to the return value(s) of the function. For functions that return no values, the type specifier (values) should be used; alternatively, :void can be used as a synonym for (values). TEST is an optional expression or predicate function which will be called with the variable result bound to the function's return value (or to a list if the function returns multiple values). The test must return non-nil or an error is signalled.
  • At toplevel within the function body, any or all of the following clauses may be included:
    • (:returns TYPE) - this declares the type of the function's return value. It should not be used it the type has been declared using the (fname -> TYPE) clause.
    • (:pre TEST+) - where TEST+ is one or more expressions or predicate function names. Each of these will be evaluated when the function is called. They must all return non-nil, or an error will be signalled.
    • (:post TEST+) - these tests are all evaluated just before the function returns, with result bound to the return value (or to a list if the function returns multiple values). They must all return non-nil, or an error will be signalled.
  • One or more arguments in a lambda list may be named _. Any such arguments will be given unique names and will be automatically declared ignored.

Examples of function definitions

Define a simple function that adds two numbers, both of which are declared to be real.

(defun* sum ((a real) (b real))
   (+ a b))

Now also declare that the function returns a real.

(defun* (sum -> real) ((a real) (b real))
   (+ a b))

Another way of declaring the function's return type.

(defun* sum ((a real) (b real))
   (:returns real)
   (+ a b))

We want to ensure that a and b are never negative. One way is to alter the type declarations:

(defun* (sum -> (real 0)) ((a (real 0)) (b (real 0)))
   (+ a b))

Another way is to define a new type:

(deftype natural () '(real 0))
(defun* (sum -> natural) ((a natural) (b natural))
 (+ a b))

Another way is to use assertions:

(defun* (sum -> real (>= result 0)) ((a real (>= a 0)) (b real (>= b 0)))
   (+ a b))


(defun* sum ((a real (>= a 0)) (b real (>= b 0)))
   (:returns real (>= result 0))
   (+ a b))

Or, using the feature that the names of single-argument predicate functions can be used as assertions:

(defun* (naturalp -> boolean) ((x real))
   (not (minusp x)))
(defun* (sum -> real naturalp) ((a real naturalp) (b real naturalp))
   (+ a b))

Another approach is to use :pre and :post clauses. Each contains one or more forms, ALL of which must evaluate to non-nil. Within :post forms, result is bound to the value that the function or form is about to return.

(defun* (sum -> real) ((a real) (b real))
   (:pre (>= a 0) (>= b 0))
   (:post (>= result 0))
   (+ a b))

Ignoring arguments

Here is a simple function that ignores all its arguments after the first:

(defun* my-first (item &rest _)

Multiple return values

A function that returns multiple values.

(defun* (floor -> (values integer integer)) ((n real) (d real))
   (cl:floor n d))

It is possible to use assertions with functions that return multiple values. When a function is declared to return multiple values, result will be bound to a list of those values.

(defun* floor ((n real) (d real))
   (:returns (values integer integer)
            (< (second result) (first result)))
   (cl:floor n d))

To declare that a function returns an unspecified number of values, of unspecified types:

(defun* (floor -> (values)) ((n real) (d real))

No return values

(defun* (print-message -> (values)) ((fmt string) &rest args)
   (apply #'format t fmt args)

The type specifier :void has been defined as a synonym for (values):

(defun* (print-message -> :void) ((fmt string) &rest args)
   (apply #'format t fmt args)

Type declarations for &rest, &optional and keyword arguments

The type of a &rest argument can be declared. The declaration refers to the types of each element in the list of arguments stored in the &rest argument.

(defun* (+ -> real) (&rest (numbers real))
   (apply #'cl:+ numbers))

A more complicated lambda list. Note that the function and its first argument do not have type declarations. Also note the syntax of typed keyword arguments:


Note that &optional arguments use the same syntax.

(defun* my-find (item (seq sequence) &key (from-end boolean)
                 ((test (or null (function (t)))) nil)
                 ((test-not (or null (function (t)))) nil)
                 ((key (or null (function (t)))) nil)
                 (start fixnum) (end fixnum))
   ...function body...)

defmethod* and defgeneric*

Here is an example of method definition. All the arguments in the arglist are normal 'specialised' arguments like you would usually find in a method definition. The form still allows you to include an assertion with each argument, however (plusp in this case).

(defmethod* (cell-value -> real) :around ((sheet <Sheet>)
                                          (x integer plusp) (y integer plusp))

Note that when you declare a return type for a method, the method body will perform type-checking, but no toplevel declaim form will be generated.

CLOS function dispatch based on classes is limited; you cannot specialise on user-defined types unless they are proper classes, for example. You may therefore sometimes want to declare that a method's argument is of a particular type, as well as declaring its class for specialisation as you normally would.

Here is an example. Note the similarity to the syntax for keyword arguments.

(defmethod* (cell-value -> real) :around ((sheet <Sheet>)
                                          ((x natural plusp) integer)
                                          ((y natural plusp) integer))

An example of defgeneric*, mainly useful to declare the return type of a set of methods. Note the documentation string can appear after the argument list, similar to defun.

(defgeneric* (cell-value -> real) (sheet x y)
  "Return the value of the cell at coordinates X,Y in SHEET.")

defgeneric* can also be used to declare types of arguments. Be careful that these don't clash with specialisers in method definitions.

(defgeneric* (cell-value -> real) (sheet (x natural) (y natural)))

defvar* and defparameter*

Used to declare the types of global variables.

(defvar* (*user-name* string) "Bob")

(defparameter* (*file-position* (integer 0)) 0)


This statement behaves like let*, with the following differences:

  • When a type is given between the variable name and expression, this is converted to a local declaration of the type of the variable, within the scope of the *let form.
  • When the form to be bound is a lambda list or cons cell, *let behaves like destructuring-bind. The value of the expression is destructured and elements are bound to the arguments in the lambda list or cons cell.
  • When the form to be bound is a list whose first element is :values, *let behaves like multiple-value-bind, using the rest of the elements in the form as the variables to be bound. Those elements may be symbols, or lists of the form (SYMBOL TYPE), in which case the bound symbol will be declared to be of the given type.
  • Any variables whose names are '_', either bare or inside a form to be destructured, will be renamed with unique symbols and declared ignored within the body. This provides a quick way to ignore arguments or parts of arguments.

For example,

(*let ((name "Bob")
       (age integer 40)
       (sex (member :male :female) :male)
       ((num street &optional suburb) address)
       ((:values (day fixnum) month year) birthday))

Expands to:

(let ((name "Bob"))
  (let ((age 40))
    (declare (integer age))
    (let ((sex :male))
      (declare ((member :male :female) sex))
          (num street &optional suburb) address
        (multiple-value-bind (day month year) birthday
          (declare (fixnum day))

Arguments can also be auto-ignored within *let statements by naming them _:

(*let (((top . _) list))
   (print top))


This statement is identical to let, except that a name comes before the binding clauses. Within the body of the nlet, this name can be called like a function, with arguments corresponding to the local variables bound in the nlet statement. Calling this function returns execution to the start of the nlet body, but the local variables will be bound to the values given to the function rather than the values specified in the nlet binding clauses.

nlet is intended to provided the same functionality as Scheme's named let. The macro does not perform tail call optimisation itself. However all modern Common Lisp implementations will perform TCO on the generated code (I am assured).

Type declaration versus type checking

Technically, declare, declaim and the like do not actually check that values stored in the associated variables conform to the declared type. They merely constitute a promise by the programmer that only values of the specified type will be stored there. The consequences of storing a string in a variable that is declared to be of type integer, are technically 'undefined'.

In practice, most modern Common Lisp implementations perform type-checking based on declaration information, especially when the safety setting is high.

Defstar allows you to force lisp to perform type checking based on declarations. If you set the global variable *check-argument-types-explicitly?* to non-nil, check-type forms will be inserted in the body of each function or method, causing an error to be raised if a value does not match its declared type.


Definitions of setf methods cannot include return type declarations in the method 'header'. The return type can still be declared using a (:returns ...) form.


(defmethod (setf (foo -> integer)) (...args...)


(defmethod (setf foo) (...args...)
   (:returns integer)                  ; legal

Syntax highlighting of Defstar macros in Emacs

Put the following code in your .emacs if you want defvar* and other forms to appear in the same face as their normal counterparts, and if you want their docstrings to also be correctly identified as docstrings rather than normal strings.

;; fontify doc strings in correct face
;; lisp-mode already fontifies 'defun*' correctly
(put 'defvar*   'doc-string-elt 3)
(put 'defparameter*   'doc-string-elt 3)
(put 'lambda*   'doc-string-elt 2)

(defvar *lisp-special-forms*
(regexp-opt '("defvar*"
              "labels*") 'words))
(font-lock-add-keywords 'lisp-mode
  `((,*lisp-special-forms* . font-lock-keyword-face)))

Dependencies (0)

    Dependents (1)

    • cl-tcod
    • GitHub
    • Quicklisp