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

   transformers	Control.Monad.Trans.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

   transformers	Control.Monad.Trans.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

   transformers	Control.Monad.Trans.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

   transformers	Control.Monad.Trans.Writer.Strict

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

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

5. mapSize :: Testable prop => (Int -> Int) -> prop -> Property

   QuickCheck	Test.QuickCheck

   Adjust the test case size for a property, by transforming it with the given function.

6. mapSubject :: (b -> a) -> SpecWith a -> SpecWith b

   hspec	Test.Hspec

   Modify the subject under test. Note that this resembles a contravariant functor on the first type parameter of SpecM. This is because the subject is passed inwards, as an argument to the spec item.

7. mapCont :: (r -> r) -> Cont r a -> Cont r a

   mtl	Control.Monad.Cont

   Apply a function to transform the result of a continuation-passing computation.

   • runCont (mapCont f m) = f . runCont
     m

8. mapError :: (MonadError e m, MonadError e' n) => (m (Either e a) -> n (Either e' b)) -> m a -> n b

   mtl	Control.Monad.Error.Class

   MonadError analogue of the mapExceptT function. The computation is unwrapped, a function is applied to the Either, and the result is lifted into the second MonadError instance.

9. mapError :: (MonadError e m, MonadError e' n) => (m (Either e a) -> n (Either e' b)) -> m a -> n b

   mtl	Control.Monad.Except

   MonadError analogue of the mapExceptT function. The computation is unwrapped, a function is applied to the Either, and the result is lifted into the second MonadError instance.

10. mapExcept :: (Either e a -> Either e' b) -> Except e a -> Except e' b

    mtl	Control.Monad.Except

    Map the unwrapped computation using the given function.

    • runExcept (mapExcept f m) = f (runExcept
      m)

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