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1. mapWriter :: ((a, w) -> (b, w')) -> Writer w a -> Writer w' b

   mtl	Control.Monad.Writer.Lazy

   Map both the return value and output of a computation using the given function.

   • runWriter (mapWriter f m) = f (runWriter
     m)

2. mapWriterT :: (m (a, w) -> n (b, w')) -> WriterT w m a -> WriterT w' n b

   mtl	Control.Monad.Writer.Lazy

   Map both the return value and output of a computation using the given function.

   • runWriterT (mapWriterT f m) = f
     (runWriterT m)

3. mapWriter :: ((a, w) -> (b, w')) -> Writer w a -> Writer w' b

   mtl	Control.Monad.Writer.Strict

   Map both the return value and output of a computation using the given function.

   • runWriter (mapWriter f m) = f (runWriter
     m)

4. mapWriterT :: (m (a, w) -> n (b, w')) -> WriterT w m a -> WriterT w' n b

   mtl	Control.Monad.Writer.Strict

   Map both the return value and output of a computation using the given function.

   • runWriterT (mapWriterT f m) = f
     (runWriterT m)

5. mapKeyVal :: (Key -> Key) -> (v1 -> v2) -> KeyMap v1 -> KeyMap v2

   aeson	Data.Aeson.KeyMap

   Transform the keys and values of a KeyMap.

6. mapMaybe :: (a -> Maybe b) -> KeyMap a -> KeyMap b

   aeson	Data.Aeson.KeyMap

   Map values and collect the Just results.

7. mapMaybeWithKey :: (Key -> v -> Maybe u) -> KeyMap v -> KeyMap u

   aeson	Data.Aeson.KeyMap

   Map values and collect the Just results.

8. mapWithKey :: (Key -> a -> b) -> KeyMap a -> KeyMap b

   aeson	Data.Aeson.KeyMap

   Map a function over all values in the map.

9. mapFromJSONKeyFunction :: (a -> b) -> FromJSONKeyFunction a -> FromJSONKeyFunction b

   aeson	Data.Aeson.Types

   Same as fmap. Provided for the consistency with ToJSONKeyFunction.

10. mapKeys :: Hashable k2 => (k1 -> k2) -> HashMap k1 v -> HashMap k2 v

    unordered-containers	Data.HashMap.Internal

    mapKeys f s is the map obtained by applying f to each key of s. The size of the result may be smaller if f maps two or more distinct keys to the same new key. In this case there is no guarantee which of the associated values is chosen for the conflicting key.

    >>> mapKeys (+ 1) (fromList [(5,"a"), (3,"b")])
    fromList [(4,"b"),(6,"a")]
    
    >>> mapKeys (\ _ -> 1) (fromList [(1,"b"), (2,"a"), (3,"d"), (4,"c")])
    fromList [(1,"c")]
    
    >>> mapKeys (\ _ -> 3) (fromList [(1,"b"), (2,"a"), (3,"d"), (4,"c")])
    fromList [(3,"c")]
    

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