Monad Translations for Higher-Order Logic

Abstract

Classical logic can be embedded into intuitionistic logic by inserting double negations in formulas. Several translations generalize this idea by using monad operators instead of double negations. They eliminate particular axioms, for instance the principle of excluded middle or the principle of explosion, and therefore can be used to embed classical logic into intuitionistic logic or intuitionistic logic into minimal logic. Such translations have been defined for first-order logic.
In this paper, we define a translation, parameterized by monad operators, for higher-order logic. In particular, the property that any formula and its translation are equivalent in the presence of the eliminated axiom holds under functional extensionality and propositional extensionality. We apply this translation to embed higher-order classical (respectively intuitionistic) logic into higher-order intuitionistic (respectively minimal) logic. By adapting Friedman’s trick, we show that coherent formulas correspond to a constructive fragment of higher-order classical logic, meaning that we can transform classical proofs into intuitionistic proofs without modifying the proven statements.