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'90	Kaleidoscope'91	Kaleidoscope'93
Constraint Evaluation	Lazy	Eager	Eager
Variables	Hold streams	Hold streams	Imperative
Concurrent Constraints	Strict	Strict	Non-strict
Syntax	Smalltalk-like	Algol-like	Algol-like
Constraint Model	Refinement	Refinement	Perturbation
Method Dispatching	Single	Multiple	Multiple
Assignment	As a constraint	As a constraint	Destructive

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;
display_number;
if old < mercury.top then
delta_grey;
elseif old > mercury.top then
delta_white;
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;