Difference between revisions of "Solution4.html"
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grandparent s = parent s >>= parent |
grandparent s = parent s >>= parent |
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</haskell> |
</haskell> |
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Latest revision as of 08:45, 9 April 2021
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