Exercises

1. Do notation
2. Combining monadic values
3. Using the List monad
4. Using the Monad class constraint

This section contains a few simple exercises to hone the reader's monadic reasoning skills and to provide a solid comprehension of the function and use of the Maybe and List monads before looking at monadic programming in more depth. The exercises will build on the previous sheep-cloning example, with which the reader should already be familiar.

Rewrite the maternalGrandfather, fathersMaternalGrandmother, and mothersPaternalGrandfather functions in Example 2 using the monadic operators return and >>=, without using any do-notation syntactic sugar.

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Write functions parent and grandparent with signature Sheep -> Maybe Sheep. They should return one sheep selected from all sheep matching the description, or Nothing if there is no such sheep. Hint: the mplus operator is useful here.

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Write functions parent and grandparent with signature Sheep -> [Sheep]. They should return all sheep matching the description, or the empty list if there is no such sheep. Hint: the mplus operator in the List monad is useful here. Also the maybeToList function in the Maybe module can be used to convert a value from the Maybe monad to the List monad.

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Monads promote modularity and code reuse by encapsulating often-used computational strategies into single blocks of code that can be used to construct many different computations. Less obviously, monads also promote modularity by allowing you to vary the monad in which a computation is done to achieve different variations of the computation. This is achieved by writing functions which are polymorphic in the monad type constructor, using the (Monad m) =>, (MonadPlus m) =>, etc. class constraints.

Write functions parent and grandparent with signature (MonadPlus m) => Sheep -> m Sheep. They should be useful in both the Maybe and List monads. How does the functions' behavior differ when used with the List monad versus the Maybe monad? If you need to review the use of type classes and class constraints in Haskell, look here.

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