Fix loop (#233)

* Update Apply superclass law

* Change ap constraint back to Monad

* Port `ap` back to Monad

* Reimplement `ap` using bind and pure

* Document why revert was needed

* Stop importing ap from Bind

* Update purescript-psa to v0.8.0

* Update "for any Bind" to "for any Monad"

* Update 'Apply - Bind' to 'Apply - Monad'

Author: JordanMartinez

package.json

@@ -7,7 +7,7 @@
   },
   "devDependencies": {
     "eslint": "^4.19.1",
-    "purescript-psa": "^0.6.0",
+    "purescript-psa": "^0.8.0",
     "pulp": "^15.0.0",
     "rimraf": "^2.6.2"
   }

src/Control/Bind.purs

@@ -6,7 +6,6 @@ module Control.Bind
   , composeKleisli, (>=>)
   , composeKleisliFlipped, (<=<)
   , ifM
-  , ap
   , module Data.Functor
   , module Control.Apply
   , module Control.Applicative
@@ -38,7 +37,7 @@ import Type.Proxy (Proxy(..), Proxy2, Proxy3)
 -- | laws:
 -- |
 -- | - Associativity: `(x >>= f) >>= g = x >>= (\k -> f k >>= g)`
--- | - Apply Superclass: `apply = ap`
+-- | - Apply Superclass: `apply f x = f >>= \f’ -> map f’ x`
 -- |
 -- | Associativity tells us that we can regroup operations which use `do`
 -- | notation so that we can unambiguously write, for example:
@@ -149,17 +148,3 @@ 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

@@ -3,6 +3,7 @@ module Control.Monad
   , liftM1
   , whenM
   , unlessM
+  , ap
   , module Data.Functor
   , module Control.Apply
   , module Control.Applicative
@@ -11,7 +12,7 @@ module Control.Monad
 
 import Control.Applicative (class Applicative, liftA1, pure, unless, when)
 import Control.Apply (class Apply, apply, (*>), (<*), (<*>))
-import Control.Bind (class Bind, bind, ap, ifM, join, (<=<), (=<<), (>=>), (>>=))
+import Control.Bind (class Bind, bind, ifM, join, (<=<), (=<<), (>=>), (>>=))
 
 import Data.Functor (class Functor, map, void, ($>), (<#>), (<$), (<$>))
 import Data.Unit (Unit)
@@ -64,3 +65,22 @@ unlessM :: forall m. Monad m => m Boolean -> m Unit -> m Unit
 unlessM mb m =  do
   b <- mb
   unless b m
+
+-- | `ap` provides a default implementation of `(<*>)` for any `Monad`, without
+-- | using `(<*>)` as provided by the `Apply`-`Monad` superclass relationship.
+-- |
+-- | `ap` can therefore be used to write `Apply` instances as follows:
+-- |
+-- | ```purescript
+-- | instance applyF :: Apply F where
+-- |   apply = ap
+-- | ```
+-- Note: Only a `Bind` constraint is needed, but this can
+-- produce loops when used with other default implementations
+-- (i.e. `liftA1`).
+-- See https://github.com/purescript/purescript-prelude/issues/232
+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')

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, ap, join, (<=<), (=<<), (>=>), (>>=))
+import Control.Bind (class Bind, bind, class Discard, discard, ifM, join, (<=<), (=<<), (>=>), (>>=))
 import Control.Category (class Category, identity)
-import Control.Monad (class Monad, liftM1, unlessM, whenM)
+import Control.Monad (class Monad, liftM1, unlessM, whenM, ap)
 import Control.Semigroupoid (class Semigroupoid, compose, (<<<), (>>>))
 
 import Data.Boolean (otherwise)