Kaleidoscope (programming language)


The Kaleidoscope programming language is a constraint programming language embedding constraints into an imperative object-oriented language. It adds keywords always, once, and assert..during to make statements about relational invariants. Objects have constraint constructors, which are not methods, to enforce the meanings of user-defined datatypes.
There are three versions of Kaleidoscope which show an evolution from declarative to an increasingly imperative style. Differences between them are as follows.
Kaleidoscope'90Kaleidoscope'91Kaleidoscope'93
Constraint EvaluationLazyEagerEager
VariablesHold streamsHold streamsImperative
Concurrent ConstraintsStrictStrictNon-strict
SyntaxSmalltalk-likeAlgol-likeAlgol-like
Constraint ModelRefinementRefinementPerturbation
Method DispatchingSingleMultipleMultiple
AssignmentAs a constraintAs a constraintDestructive

Example

Compare the two code segments, both of which allow a user to drag the level of mercury in a simple graphical thermometer with the mouse.
Without constraints:
while mouse.button = down do
old <- mercury.top;
mercury.top <- mouse.location.y;
temperature <- mercury.height / scale;
if old < mercury.top then
delta_grey;
display_number;
elseif old > mercury.top then
delta_white;
display_number;
end if;
end while;
With constraints:
always: temperature = mercury.height / scale;
always: white rectangle;
always: grey rectangle;
always: display number;
while mouse.button = down do
mercury.top = mouse.location.y;
end while;