CARTESIAN-PRODUCT-SWITCH is a macro for choosing the appropriate form to execute according to the combined results of multiple tests. This is a straightforward and efficient alternative to the convoluted ad-hoc conditionals one might otherwise resort to.

cartesian-product-switch

Project's home: http://www.hexstreamsoft.com/projects/cartesian-product-switch/


CARTESIAN-PRODUCT-SWITCH is a macro for choosing the appropriate form
to execute according to the combined results of multiple tests. This is a
straightforward and efficient alternative to the convoluted ad-hoc
conditionals one might otherwise resort to.

cartesian-product-switch was born as cartesian-product-case but a
fairly obvious limitation was soon discovered, prompting a redesign
and library name change. Further features and extensibility were also
added.


Here's an example:

(let ((manner :flip) (direction :backward))
  (cartesian-product-switch ((case manner
                               :walk
                               :flip)
                             (case direction
                               :forward
                               :in-place
                               :backward))
    "Walking..."
    (error "Walking in-place is too boring!")
    "Awkwardly walking backwards..."
    "Frontflip!"
    "Jumping in-place."
    "Backflip!"
    (t (error "Don't know how to ~A while going ~A." manner direction))))


Currently supported testclause kinds:
(case ccase ecase typecase ctypecase etypecase if cond svref)

It's also possible to make your own kinds.
Check definitions.lisp in particular for inspiration.
In a nutshell, call (cartesian-product-switch:define operator (args) body),
where BODY must return 3 values:

SELECTION-FORM:
A form evaluating to a non-negative integer less than SELECTION-COUNT.

SELECTION-COUNT:
The number of possibilities this testclause introduces.

DEFAULTP:
If true, then the SELECTION-FORM can evaluate to NIL to trigger the "default".


This library is in the Public Domain.
See the UNLICENSE file for details.


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