Deprecate MonadZero (#64)

Author: kl0tl

src/Control/Alternative.purs

@@ -1,5 +1,6 @@
 module Control.Alternative
   ( class Alternative
+  , guard
   , module Control.Alt
   , module Control.Applicative
   , module Control.Apply
@@ -12,6 +13,7 @@ import Control.Applicative (class Applicative, pure, liftA1, unless, when)
 import Control.Apply (class Apply, apply, (*>), (<*), (<*>))
 import Control.Plus (class Plus, empty)
 
+import Data.Unit (Unit, unit)
 import Data.Functor (class Functor, map, void, ($>), (<#>), (<$), (<$>))
 
 -- | The `Alternative` type class has no members of its own; it just specifies
@@ -25,3 +27,24 @@ import Data.Functor (class Functor, map, void, ($>), (<#>), (<$), (<$>))
 class (Applicative f, Plus f) <= Alternative f
 
 instance alternativeArray :: Alternative Array
+
+-- | Fail using `Plus` if a condition does not hold, or
+-- | succeed using `Applicative` if it does.
+-- |
+-- | For example:
+-- |
+-- | ```purescript
+-- | import Prelude
+-- | import Control.Alternative (guard)
+-- | import Data.Array ((..))
+-- |
+-- | factors :: Int -> Array Int
+-- | factors n = do
+-- |   a <- 1..n
+-- |   b <- 1..n
+-- |   guard $ a * b == n
+-- |   pure a
+-- | ```
+guard :: forall m. Alternative m => Boolean -> m Unit
+guard true = pure unit
+guard false = empty

src/Control/MonadPlus.purs

@@ -12,20 +12,23 @@ module Control.MonadPlus
   ) where
 
 import Control.Alt (class Alt, alt, (<|>))
-import Control.Alternative (class Alternative)
+import Control.Alternative (class Alternative, guard)
 import Control.Applicative (class Applicative, pure, liftA1, unless, when)
 import Control.Apply (class Apply, apply, (*>), (<*), (<*>))
 import Control.Bind (class Bind, bind, ifM, join, (<=<), (=<<), (>=>), (>>=))
 import Control.Monad (class Monad, ap, liftM1)
-import Control.MonadZero (class MonadZero, guard)
+import Control.MonadZero (class MonadZero)
 import Control.Plus (class Plus, empty)
 
 import Data.Functor (class Functor, map, void, ($>), (<#>), (<$), (<$>))
 
--- | The `MonadPlus` type class has no members of its own but extends
--- | `MonadZero` with an additional law:
+-- | The `MonadPlus` type class has no members of its own; it just specifies
+-- | that the type has both `Monad` and `Alternative` instances.
+-- |
+-- | Types which have `MonadPlus` instances should also satisfy the following
+-- | law:
 -- |
 -- | - Distributivity: `(x <|> y) >>= f == (x >>= f) <|> (y >>= f)`
-class MonadZero m <= MonadPlus m
+class (Monad m, Alternative m) <= MonadPlus m
 
 instance monadPlusArray :: MonadPlus Array

src/Control/MonadZero.purs

@@ -1,6 +1,15 @@
+-- | This module is **deprecated** and will be removed in a future release.
+-- |
+-- | The annihilation law witnessed by `MonadZero` is trivially satisfied by
+-- | lawful monads due to parametricity: while evaluating `empty >>= f`, the
+-- | function `f` can’t ever be called, since that would require `empty` to
+-- | produce a value, which means that `empty >>= f` must be the same as
+-- | `empty >>= pure`, which by the monad laws is just `empty`.
+-- |
+-- | Use `Monad` and `Alternative` constraints instead.
+
 module Control.MonadZero
   ( class MonadZero
-  , guard
   , module Control.Alt
   , module Control.Alternative
   , module Control.Applicative
@@ -12,15 +21,16 @@ module Control.MonadZero
   ) where
 
 import Control.Alt (class Alt, alt, (<|>))
-import Control.Alternative (class Alternative)
+import Control.Alternative (class Alternative, guard)
 import Control.Applicative (class Applicative, pure, liftA1, unless, when)
 import Control.Apply (class Apply, apply, (*>), (<*), (<*>))
 import Control.Bind (class Bind, bind, ifM, join, (<=<), (=<<), (>=>), (>>=))
 import Control.Monad (class Monad, ap, liftM1)
 import Control.Plus (class Plus, empty)
 
 import Data.Functor (class Functor, map, void, ($>), (<#>), (<$), (<$>))
-import Data.Unit (Unit, unit)
+
+import Prim.TypeError (class Warn, Text)
 
 -- | The `MonadZero` type class has no members of its own; it just specifies
 -- | that the type has both `Monad` and `Alternative` instances.
@@ -29,28 +39,4 @@ import Data.Unit (Unit, unit)
 -- | laws:
 -- |
 -- | - Annihilation: `empty >>= f = empty`
-class (Monad m, Alternative m) <= MonadZero m
-
-instance monadZeroArray :: MonadZero Array
-
--- | Fail using `Plus` if a condition does not hold, or
--- | succeed using `Monad` if it does.
--- |
--- | For example:
--- |
--- | ```purescript
--- | import Prelude
--- | import Control.Monad (bind)
--- | import Control.MonadZero (guard)
--- | import Data.Array ((..))
--- |
--- | factors :: Int -> Array Int
--- | factors n = do
--- |   a <- 1..n
--- |   b <- 1..n
--- |   guard $ a * b == n
--- |   pure a
--- | ```
-guard :: forall m. MonadZero m => Boolean -> m Unit
-guard true = pure unit
-guard false = empty
+class (Monad m, Alternative m, Warn (Text "'MonadZero' is deprecated, use 'Monad' and 'Alternative' constraints instead")) <= MonadZero m