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How does Haskell know which is correct monad instance for each return expression?
newtype MaybeT m a = MaybeT { runMaybeT :: m (Maybe a) }
instance Monad m => Monad (MaybeT m) where
return = MaybeT . return . return
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A typeclass defines a set of methods that is shared across multiple types. For a type to belong to a typeclass, it needs to implement the methods of that typeclass. These implementations are ad-hoc: methods can have different implementations for different types.
Type and data type refer to exactly the same concept. The Haskell keywords type and data are different, though: data allows you to introduce a new algebraic data type, while type just makes a type synonym. See the Haskell wiki for details.
In Haskell, every statement is considered as a mathematical expression and the category of this expression is called as a Type. You can say that "Type" is the data type of the expression used at compile time. To learn more about the Type, we will use the ":t" command.
Making your own data type in HaskellNew data types are created via the data keyword. To create a Point data type, we need to provide a type constructor (the name of our type) and a data constructor (used to construct new instances of the type), followed by the types our type will contain.
It's actually unambiguous from the context.
Let's play the typechecker,
-- From the signature
MaybeT . return . return :: a -> MaybeT a
-- From the type of MaybeT
return . return :: a -> m (Maybe a)
-- From the type of `.`
(return :: Maybe a -> m a) . (return :: a -> Maybe a)
And once we have the type of each return, the "instance selection algorithm" will correctly choose the first to use ms return and the second to be Maybe.
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