Programming with Propositions

A proposition is a statement expressing a factual claim, like "two plus two equals four." In Coq, propositions are written as expressions of type Prop. Although we haven't mentioned it explicitly, we have already seen numerous examples.

Both provable and unprovable claims are perfectly good propositions. Simply being a proposition is one thing; being provable is something else!

Both 2 + 2 = 4 and 2 + 2 = 5 are legal expressions of type Prop. We've seen one place that propositions can appear in Coq: in Theorem (and Lemma and Example) declarations.

But they can be used in many other ways. For example, we can give a name to a proposition using a Definition, just as we have given names to expressions of other sorts.

We can later use this name in any situation where a proposition is expected — for example, as the claim in a Theorem declaration.

We've seen several ways of constructing propositions.

• We can define a new proposition primitively using Inductive.
• Given two expressions e1 and e2 of the same type, we can form the proposition e1 = e2, which states that their values are equal.
• We can combine propositions using implication and quantification.

We have also seen parameterized propositions, such as even and beautiful.

The type of even, i.e., nat→Prop, can be pronounced in three equivalent ways: (1) "even is a function from numbers to propositions," (2) "even is a family of propositions, indexed by a number n," or (3) "even is a property of numbers." Propositions — including parameterized propositions — are first-class citizens in Coq. For example, we can define functions from numbers to propositions...

... and then partially apply them:

We can even pass propositions — including parameterized propositions — as arguments to functions:

Here are two more examples of passing parameterized propositions as arguments to a function. The first function, true_for_all_numbers, takes a proposition P as argument and builds the proposition that P is true for all natural numbers.

The second, preserved_by_S, takes P and builds the proposition that, if P is true for some natural number n', then it is also true by the successor of n' — i.e. that P is preserved by successor:

Exercise: 3 stars (combine_odd_even)

Complete the definition of the combine_odd_even function below. It takes as arguments two properties of numbers Podd and Peven. As its result, it should return a new property P such that P n is equivalent to Podd n when n is odd, and equivalent to Peven n otherwise.

To test your definition, see whether you can prove the following facts:

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One more quick digression, for adventurous souls: if we can define parameterized propositions using Definition, then can we also define them using Fixpoint? Of course we can! However, this kind of "recursive parameterization" doesn't correspond to anything very familiar from everyday mathematics. The following exercise gives a slightly contrived example.

Exercise: 4 stars, optional (true_upto_n__true_everywhere)

Define a recursive function true_upto_n__true_everywhere that makes true_upto_n_example work.

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