List-Types October 29th, 2015
Patrick Stein



I often find myself wanting to declare a type as a list of integers or a list of symbols or some such thing. My natural inclination is to do this:

(deftype list-of (&optional (type '*))
  `(or null
       (cons ,type (list-of ,type))))

However, the Common Lisp spec requires that all type declaration expansions terminate. As such, one has to hop through various hoops to use a SATISFIES clause in a DEFTYPE to do that. This package wraps that technique and defines the types:

(sequence-of &optional (type '*) (min-elements '*) (max-elements '*)
(sequence-of* &optional (type '*))
(sequence-of+ &optional (type '*))

(list-of &optional (type '*) (min-elements '*) (max-elements '*))
(list-of* &optional (type '*))
(list-of+ &optional (type '*))

Additionally, this package also exports the ALEXANDRIA types for PROPER-LIST and CIRCULAR-LIST.

The MIN-ELEMENTS and MAX-ELEMENTS can be either a non-negative integer or the symbol *.

There is no attempt to ensure that a specified MIN-ELEMENTS is less than MAX-ELEMENTS.

Note: there is currently no support for CIRCULAR-LIST-OF.


Unfortunately, this is not sufficient unless your code gets recompiled every time. If you have a block of code like this:

(defun foo (bar)
  (check-type bar (list-of+ integer))

This will work when you compile and load it. It will not work when you just load it because at compile time, the expansion of list-of+ created a predicate that will not be there if you just load the compiled check-type into a clean image.

The ugly workaround for this is to use one of the macros before the check-type:

(ensure-sequence-type &optional (type '*)
                                (min-elements '*)
                                (max-elements '*))
(ensure-sequence-type* &optional (type '*))
(ensure-sequence-type+ &optional (type '*))

So, your code will look like one of the following:

(ensure-sequence-type+ integer)
(defun foo (bar)
  (check-type bar (list-of+ integer))

;;; Or

(defun foo (bar)
  (ensure-sequence-type+ integer)
  (check-type bar (list-of+ integer))

Grid-Generators v0.4.20151016 (and List-Types v0.4.20151029) Released October 29th, 2015
Patrick Stein

Two new Common Lisp libraries released: GRID-GENERATORS and LIST-TYPES. GRID-GENERATORS and GRID-ITERATE packages I often find myself wanting to iterate over the points in a rectangular region of a grid in an arbitrary number of dimensions. If my grid is only one-dimensional or two-dimensional then I often just write nested loops to traverse the points […]

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