Work items can only have one kind of relationship to another (parent/child, blocking/blocked by/related).

These can then be used to develop a definition that states the essence of the living thing—what makes it what it is and thus cannot be altered; the essence is, of course, immutable.

The Aristotelian method dominated classification until the 19th century. His scheme was, in effect, that the classification of a living thing by its nature—i.e., what it really is, as against superficial resemblances—requires the examination of many specimens, the discarding of variable characters (since they must be accidental, not essential), and the establishment of constant characters.

Something that you're trying to keep the same, in order to achieve goal X (such as a "log lookup time" above).

An invariant is like a rule or an assumption that can be used to dictate the logic of your program.

An invariant (in common sense) means some conditions that must be true at some point in time or even always while your program is executing. e.g. PreConditions and PostConditions can be used to assert some conditions that must be true when a function is called and when it returns. Object invariants can be used to assert that a object must have a valid state throughout the time it exists. This is the design by contract principle.

Modern cars, however, use a single stick that pivots around among the gears. It's designed in such a way that, on a modern stick-shift car, it is not possible to engage two gears at the same time.

In OOP, an invariant is a set of assertions that must always hold true during the life of an object for the program to be valid. It should hold true from the end of the constructor to the start of the destructor whenever the object is not currently executing a method that changes its state.

42 View upvote and downvote totals. This answer is not useful Save this answer. Show activity on this post. It is a condition you know to always be true at a particular place in your logic

The age of a parent is greater than the age of their biological children.

an invariant is something like of a fixed relationship between varying entities. For example, your age will always be less than that compared to your biological parents. Both your age, and your parent's age changes in the passage of time, but the relationship that i mentioned above is an invariant.

For instance, a binary search tree might have the invariant that for every node, the key of the node's left child is less than the node's own key. A correctly written insertion function for this tree will maintain that invariant. As you can tell, that's not the sort of thing you can store in a variable: it's more a statement about the program. By figuring out what sort of invariants your program should maintain, then reviewing your code to make sure that it actually maintains those invariants, you can avoid logical errors in your code.

Final Form makes the assumption that your validation functions are "pure" or "idempotent", i.e. will always return the same result when given the same values. This is why it doesn't run the synchronous validation again (just to double check) before allowing the submission: because it's already stored the results of the last time it ran it.

invariant