# Library iris.bi.extensions

This file defines various extensions to the base BI interface, via
typeclasses that BIs can optionally implement.

From iris.bi Require Export derived_connectives.

From iris.prelude Require Import options.

Class BiAffine (PROP : bi) := absorbing_bi (Q : PROP) : Affine Q.

Global Hint Mode BiAffine ! : typeclass_instances.

Global Existing Instance absorbing_bi | 0.

Class BiPositive (PROP : bi) :=

bi_positive (P Q : PROP) : <affine> (P ∗ Q) ⊢ <affine> P ∗ Q.

Global Hint Mode BiPositive ! : typeclass_instances.

The class BiLöb is required for the iLöb tactic. However, for most BI
logics BiLaterContractive should be used, which gives an instance of BiLöb
automatically (see derived_laws_later). A direct instance of BiLöb is useful
when considering a BI logic with a discrete OFE, instead of an OFE that takes
step-indexing of the logic in account.
The internal/"strong" version of Löb (▷ P → P) ⊢ P is derivable from BiLöb.
It is provided by the lemma löb in derived_laws_later.

Class BiLöb (PROP : bi) :=

löb_weak (P : PROP) : (▷ P ⊢ P) → (True ⊢ P).

Global Hint Mode BiLöb ! : typeclass_instances.

Global Arguments löb_weak {_ _} _ _.

Notation BiLaterContractive PROP :=

(Contractive (bi_later (PROP:=PROP))) (only parsing).

löb_weak (P : PROP) : (▷ P ⊢ P) → (True ⊢ P).

Global Hint Mode BiLöb ! : typeclass_instances.

Global Arguments löb_weak {_ _} _ _.

Notation BiLaterContractive PROP :=

(Contractive (bi_later (PROP:=PROP))) (only parsing).

The class BiPureForall states that universal quantification commutes with
the embedding of pure propositions. The reverse direction of the entailment
described by this type class is derivable, so it is not included.
An instance of BiPureForall itself is derivable if we assume excluded middle
in Coq, see the lemma bi_pure_forall_em in derived_laws.