Implement ap in terms of Bind (#229)

Co-authored-by: Thomas Honeyman <[email protected]>

Author: hdgarrood

src/Control/Bind.purs

@@ -6,6 +6,7 @@ module Control.Bind
   , composeKleisli, (>=>)
   , composeKleisliFlipped, (<=<)
   , ifM
+  , ap
   , module Data.Functor
   , module Control.Apply
   , module Control.Applicative
@@ -32,10 +33,11 @@ import Data.Unit (Unit)
 -- |
 -- | where the function argument of `f` is given the name `y`.
 -- |
--- | Instances must satisfy the following law in addition to the `Apply`
+-- | Instances must satisfy the following laws in addition to the `Apply`
 -- | laws:
 -- |
 -- | - Associativity: `(x >>= f) >>= g = x >>= (\k -> f k >>= g)`
+-- | - Apply Superclass: `apply = ap`
 -- |
 -- | Associativity tells us that we can regroup operations which use `do`
 -- | notation so that we can unambiguously write, for example:
@@ -134,3 +136,17 @@ infixr 1 composeKleisliFlipped as <=<
 -- | ```
 ifM :: forall a m. Bind m => m Boolean -> m a -> m a -> m a
 ifM cond t f = cond >>= \cond' -> if cond' then t else f
+
+-- | `ap` provides a default implementation of `(<*>)` for any `Bind`, without
+-- | using `(<*>)` as provided by the `Apply`-`Bind` superclass relationship.
+-- |
+-- | `ap` can therefore be used to write `Apply` instances as follows:
+-- |
+-- | ```purescript
+-- | instance applyF :: Apply F where
+-- |   apply = ap
+-- | ```
+ap :: forall m a b. Bind m => m (a -> b) -> m a -> m b
+ap f a = do
+  f' <- f
+  map f' a

src/Control/Monad.purs

@@ -1,7 +1,6 @@
 module Control.Monad
   ( class Monad
   , liftM1
-  , ap
   , whenM
   , unlessM
   , module Data.Functor
@@ -12,7 +11,7 @@ module Control.Monad
 
 import Control.Applicative (class Applicative, liftA1, pure, unless, when)
 import Control.Apply (class Apply, apply, (*>), (<*), (<*>))
-import Control.Bind (class Bind, bind, ifM, join, (<=<), (=<<), (>=>), (>>=))
+import Control.Bind (class Bind, bind, ap, ifM, join, (<=<), (=<<), (>=>), (>>=))
 
 import Data.Functor (class Functor, map, void, ($>), (<#>), (<$), (<$>))
 import Data.Unit (Unit)
@@ -27,7 +26,6 @@ import Data.Unit (Unit)
 -- |
 -- | - Left Identity: `pure x >>= f = f x`
 -- | - Right Identity: `x >>= pure = x`
--- | - Applicative Superclass: `apply = ap`
 class (Applicative m, Bind m) <= Monad m
 
 instance monadFn :: Monad ((->) r)
@@ -50,23 +48,6 @@ liftM1 f a = do
   a' <- a
   pure (f a')
 
--- | `ap` provides a default implementation of `(<*>)` for any
--- | [`Monad`](#monad), without using `(<*>)` as provided by the
--- | [`Apply`](#apply)-[`Monad`](#monad) superclass relationship.
--- |
--- | `ap` can therefore be used to write [`Apply`](#apply) instances as
--- | follows:
--- |
--- | ```purescript
--- | instance applyF :: Apply F where
--- |   apply = ap
--- | ```
-ap :: forall m a b. Monad m => m (a -> b) -> m a -> m b
-ap f a = do
-  f' <- f
-  a' <- a
-  pure (f' a')
-
 -- | Perform a monadic action when a condition is true, where the conditional
 -- | value is also in a monadic context.
 whenM :: forall m. Monad m => m Boolean -> m Unit -> m Unit

src/Prelude.purs

@@ -30,9 +30,9 @@ module Prelude
 
 import Control.Applicative (class Applicative, pure, liftA1, unless, when)
 import Control.Apply (class Apply, apply, (*>), (<*), (<*>))
-import Control.Bind (class Bind, bind, class Discard, discard, ifM, join, (<=<), (=<<), (>=>), (>>=))
+import Control.Bind (class Bind, bind, class Discard, discard, ifM, ap, join, (<=<), (=<<), (>=>), (>>=))
 import Control.Category (class Category, identity)
-import Control.Monad (class Monad, ap, liftM1, unlessM, whenM)
+import Control.Monad (class Monad, liftM1, unlessM, whenM)
 import Control.Semigroupoid (class Semigroupoid, compose, (<<<), (>>>))
 
 import Data.Boolean (otherwise)