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structure Std.Iterators.PostconditionT (m : Type w → Type w') (α : Type w) : Type (max w w')

PostconditionT m α represents an operation in the monad m together with a intrinsic proof that some postcondition holds for the α valued monadic result. It consists of a predicate P about α and an element of m ({ a // P a }) and is a helpful tool for intrinsic verification, notably termination proofs, in the context of iterators.

PostconditionT m is a monad if m is. However, note that PostconditionT m α is a structure, so that the compiler will generate inefficient code from recursive functions returning PostconditionT m α. Optimizations for ReaderT, StateT etc. aren't applicable for structures.

Moreover, PostconditionT m α is not a well-behaved monad transformer because PostconditionT.lift neither commutes with pure nor with bind.

• Property : α → Prop

  A predicate that holds for the return value(s) of the m-monadic operation.

• operation : m (Subtype self.Property)

  The actual monadic operation. Its return value is bundled together with a proof that it satisfies Property.

@[inline]

def Std.Iterators.PostconditionT.lift {α : Type w} {m : Type w → Type w'} [Functor m] (x : m α) : PostconditionT m α

Lifts an operation from m to PostconditionT m without asserting any nontrivial postcondition.

Caution: lift is not a lawful lift function. For example, pure a : PostconditionT m α is not the same as PostconditionT.lift (pure a : m α).

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def Std.Iterators.PostconditionT.liftWithProperty {α : Type w} {m : Type w → Type w'} {P : α → Prop} (x : m { α : α // P α }) : PostconditionT m α

Lifts a monadic value from m { a : α // P a } to a value PostconditionT m α.

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def Std.Iterators.PostconditionT.map {m : Type w → Type w'} [Functor m] {α β : Type w} (f : α → β) (x : PostconditionT m α) : PostconditionT m β

Given a function f : α → β, returns a a function PostconditionT m α → PostconditionT m β, turning PostconditionT m into a functor.

The postcondition of the x.map f states that the return value is the image under f of some a : α satisfying the x.Property.

@[inline]

def Std.Iterators.PostconditionT.bind {m : Type w → Type w'} [Monad m] {α β : Type w} (x : PostconditionT m α) (f : α → PostconditionT m β) : PostconditionT m β

Given a function α → PostconditionT m β, returns a a function PostconditionT m α → PostconditionT m β, turning PostconditionT m into a monad.

@[inline]

def Std.Iterators.PostconditionT.pbind {m : Type w → Type w'} [Monad m] {α β : Type w} (x : PostconditionT m α) (f : Subtype x.Property → PostconditionT m β) : PostconditionT m β

A version of bind that provides a proof of the previous postcondition to the mapping function.

@[inline]

def Std.Iterators.PostconditionT.liftMap {m : Type w → Type w'} [Monad m] {α β : Type w} (f : α → β) (x : m α) : PostconditionT m β

Lifts an operation from m to PostConditionT m and then applies PostconditionT.map.

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def Std.Iterators.PostconditionT.run {m : Type w → Type w'} [Monad m] {α : Type w} (x : PostconditionT m α) : m α

Converts an operation from PostConditionT m to m, discarding the postcondition.

instance Std.Iterators.instFunctorPostconditionT {m : Type w → Type w'} [Functor m] : Functor (PostconditionT m)

instance Std.Iterators.instMonadPostconditionT {m : Type w → Type w'} [Monad m] : Monad (PostconditionT m)

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theorem Std.Iterators.PostconditionT.computation_pure {m : Type w → Type w'} [Monad m] {α : Type w} {x : α} : (pure x).operation = pure ⟨x, ⋯⟩

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theorem Std.Iterators.PostconditionT.property_pure {m : Type w → Type w'} [Monad m] {α : Type w} {x : α} : (pure x).Property = fun (x_1 : α) => x = x_1

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theorem Std.Iterators.PostconditionT.map_pure {m : Type w → Type w'} [Monad m] [LawfulMonad m] {α β : Type w} {f : α → β} {a : α} : PostconditionT.map f (pure a) = pure (f a)

@[simp]

theorem Std.Iterators.PostconditionT.property_map {m : Type w → Type w'} [Functor m] {α β : Type w} {x : PostconditionT m α} {f : α → β} {b : β} : (PostconditionT.map f x).Property b ↔ ∃ (a : α), f a = b ∧ x.Property a

@[simp]

theorem Std.Iterators.PostconditionT.operation_map {m : Type w → Type w'} [Functor m] {α β : Type w} {x : PostconditionT m α} {f : α → β} : (PostconditionT.map f x).operation = (fun (a : Subtype x.Property) => ⟨f a.val, ⋯⟩) <$> x.operation

@[simp]

theorem Std.Iterators.PostconditionT.operation_lift {m : Type w → Type w'} [Functor m] {α : Type w} {x : m α} : (lift x).operation = (fun (x_1 : α) => ⟨x_1, True.intro⟩) <$> x