module Prelude.Applicative

import Builtins

import Prelude.Basics
import Prelude.Bool
import Prelude.Classes
import Prelude.Functor

---- Applicative functors/Idioms

infixl 2 <$>

class Functor f => Applicative (f : Type -> Type) where
    pure  : a -> f a
    (<$>) : f (a -> b) -> f a -> f b

instance Applicative id where
    pure a = a
    f <$> a = f a

class (Applicative f, VerifiedFunctor f) => VerifiedApplicative (f : Type -> Type) where
  applicativeMap : (x : f a) -> (g : a -> b) ->
                   map g x = pure g <$> x
  applicativeIdentity : (x : f a) -> pure id <$> x = x
  applicativeComposition : (x : f a) -> (g1 : f (a -> b)) -> (g2 : f (b -> c)) ->
                           ((pure (.) <$> g2) <$> g1) <$> x = g2 <$> (g1 <$> x)
  applicativeHomomorphism : (x : a) -> (g : a -> b) ->
                            (<$>) {f} (pure g) (pure x) = pure {f} (g x)
  applicativeInterchange : (x : a) -> (g : f (a -> b)) ->
                           g <$> pure x = pure (\g' : a -> b => g' x) <$> g

infixl 2 <$
(<$) : Applicative f => f a -> f b -> f a
a <$ b = map const a <$> b

infixl 2 $>
($>) : Applicative f => f a -> f b -> f b
a $> b = map (const id) a <$> b

||| Lift a function to an applicative
liftA : Applicative f => (a -> b) -> f a -> f b
liftA f a = pure f <$> a

||| Lift a two-argument function to an applicative
liftA2 : Applicative f => (a -> b -> c) -> f a -> f b -> f c
liftA2 f a b = (map f a) <$> b

||| Lift a three-argument function to an applicative
liftA3 : Applicative f => (a -> b -> c -> d) -> f a -> f b -> f c -> f d
liftA3 f a b c = (map f a) <$> b <$> c

infixl 3 <|>
class Applicative f => Alternative (f : Type -> Type) where
    empty : f a
    (<|>) : f a -> f a -> f a

||| `guard a` is `pure ()` if `a` is `True` and `empty` if `a` is `False`
guard : Alternative f => Bool -> f ()
guard a = if a then pure () else empty

||| Conditionally execute an applicative expression
when : Applicative f => Bool -> Lazy (f ()) -> f ()
when True f = Force f
when False f = pure ()