Add ComonadAsk

Author: garyb

src/Control/Comonad/Env/Class.purs

@@ -7,11 +7,20 @@ import Control.Comonad.Env.Trans (EnvT(..))
 
 import Data.Tuple (Tuple(..), fst)
 
--- | The `ComonadEnv` type class represents those monads which support a global environment via
--- | `ask` and `local`.
+-- | The `ComonadEnv` type class represents those comonads which support a
+-- | global environment that can be provided via the `ask` function.
 -- |
--- | - `ask` reads the current environment from the context.
--- | - `local` changes the value of the global environment.
+-- | An implementation is provided for `EnvT`.
+class Comonad w <= ComonadAsk e w | w -> e where
+  ask :: forall a. w a -> e
+
+-- | Get a value which depends on the environment.
+asks :: forall e1 e2 w. ComonadEnv e1 w => (e1 -> e2) -> w e1 -> e2
+asks f x = f (ask x)
+
+-- | The `ComonadEnv` type class extends `ComonadAsk` with a function
+-- | `local f x` that allows the value of the local context to be modified for
+-- | the duration of the execution of action `x`.
 -- |
 -- | An implementation is provided for `EnvT`.
 -- |
@@ -20,19 +29,18 @@ import Data.Tuple (Tuple(..), fst)
 -- | - `ask (local f x) = f (ask x)`
 -- | - `extract (local _ x) = extract a`
 -- | - `extend g (local f x) = extend (g <<< local f) x`
-class Comonad w <= ComonadEnv e w | w -> e where
-  ask :: forall a. w a -> e
+class ComonadAsk e w <= ComonadEnv e w | w -> e where
   local :: forall a. (e -> e) -> w a -> w a
 
--- | Get a value which depends on the environment.
-asks :: forall e1 e2 w. ComonadEnv e1 w => (e1 -> e2) -> w e1 -> e2
-asks f x = f (ask x)
+instance comonadAskTuple :: ComonadAsk e (Tuple e) where
+  ask = fst
 
 instance comonadEnvTuple :: ComonadEnv e (Tuple e) where
-  ask = fst
   local f (Tuple x y) = Tuple (f x) y
 
-instance comonadEnvEnvT :: Comonad w => ComonadEnv e (EnvT e w) where
+instance comonadAskEnvT :: Comonad w => ComonadAsk e (EnvT e w) where
   ask (EnvT x) = fst x
+
+instance comonadEnvEnvT :: Comonad w => ComonadEnv e (EnvT e w) where
   local f (EnvT x) = EnvT case x of
     Tuple x y -> Tuple (f x) y