Heyting

Heyting algebras are bounded lattices that are also equipped with an additional binary operation imp (for implication, also written as →).

Implication obeys the following laws:

• a → a = 1
• a ∧ (a → b) = a ∧ b
• b ∧ (a → b) = b
• a → (b ∧ c) = (a → b) ∧ (a → c)

In heyting algebras, and is equivalent to meet and or is equivalent to join; both methods are available.

Heyting algebra also define complement operation (sometimes written as ¬a). The complement of a is equivalent to (a → 0), and the following laws hold:

• a ∧ ¬a = 0

However, in Heyting algebras this operation is only a pseudo-complement, since Heyting algebras do not necessarily provide the law of the excluded middle. This means that there is no guarantee that (a ∨ ¬a) = 1.

Heyting algebras model intuitionistic logic. For a model of classical logic, see the boolean algebra type class implemented as Bool.