Personal tools

Solution4.html

From HaskellWiki

(Difference between revisions)
Jump to: navigation, search
(New page: <haskell> parent :: MonadPlus m => Sheep -> m Sheep parent s = (toMonad (father s)) `mplus` (toMonad (mother s)) grandparent :: MonadPlus m => Sheep -> m Sheep grandparent s = (toMonad (p...)
 
(Disclaimer for the alternative solution)
 
(One intermediate revision by one user not shown)
Line 15: Line 15:
   
 
If the compiler cannot guess which MonadPlus to use you will need to specify it when the function is called. So, <hask>parent someSheep :: Maybe Sheep</hask> will use the Maybe monad and either <hask>parent someSheep :: [] Sheep</hask> or <hask>parent someSheep :: [Sheep]</hask> will use the list monad.
 
If the compiler cannot guess which MonadPlus to use you will need to specify it when the function is called. So, <hask>parent someSheep :: Maybe Sheep</hask> will use the Maybe monad and either <hask>parent someSheep :: [] Sheep</hask> or <hask>parent someSheep :: [Sheep]</hask> will use the list monad.
  +
  +
  +
This next alternative grandparent function only works in the case of the List monad (see exercise 5.2 for why the Maybe monad does not work):
  +
<haskell>
  +
grandparent :: (MonadPlus m) => Sheep -> m Sheep
  +
grandparent s = parent s >>= parent
  +
</haskell>

Latest revision as of 17:56, 10 June 2013

parent :: MonadPlus m => Sheep -> m Sheep
parent s = (toMonad (father s)) `mplus` (toMonad (mother s))
 
grandparent :: MonadPlus m => Sheep -> m Sheep
grandparent s = (toMonad (parentalGrandfather s)) `mplus`
                (toMonad (parentalGrandmother s)) `mplus`
                (toMonad (maternalGrandfather s)) `mplus`
                (toMonad (maternalGrandmother s))
 
toMonad :: MonadPlus m => Maybe a -> m a
toMonad Nothing = mzero
toMonad (Just s) = return s
If the compiler cannot guess which MonadPlus to use you will need to specify it when the function is called. So,
parent someSheep :: Maybe Sheep
will use the Maybe monad and either
parent someSheep :: [] Sheep
or
parent someSheep :: [Sheep]
will use the list monad.


This next alternative grandparent function only works in the case of the List monad (see exercise 5.2 for why the Maybe monad does not work):

grandparent :: (MonadPlus m) => Sheep -> m Sheep
grandparent s = parent s >>= parent