Re: [Haskell-beginners] Actions v. Functions

On Sat, Oct 19, 2013 at 07:03:22PM -0700, Rein Henrichs wrote:

An action in this sense is just a function of type `Monad m => a -> m b' (or one where the monad is specified, like `a -> IO b'). This type signature fully specifies the "formal properties" of an action and shows that actions do not differ from functions: they are merely functions of a certain type.

I do not think so. An action is any value of a monadic type Monad m => m a An action is just a computation in a monad. When executed, an action of type `Monad m => m a' performs a computation and returns a value of type `a'. The nature of the computation depends on the monad being considered. For instance, actions in the IO monad, when executed, interacts with the system (to do IO, for instance). Actions in the Maybe monad may fail or succeed. Actions in the Reader monad can access an environment carrying some information. For some monads, an action may be a function, like in the Reader, Writer and State monads. But that is not a requirement. Actions in the Maybe and list monads are not functions.

On Sat, Oct 19, 2013 at 3:23 PM, Brandon Allbery wrote:

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On Sat, Oct 19, 2013 at 5:09 PM, MJ Williams < matthewjwilliams101@gmail.com> wrote:

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For example, `main = do input <- getLine ...', where `getLine' is an action as distinguished from a function such as `main'. Other examples of actions in Haskell include `print', `putStrLn' and

getLine and main are both actions in the IO monad.

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Actions aren't so much a formal thing as a conceptual "handle" to help with understanding monads. In particular, what a particular action represents depends on the monad, and in some cases on the particular implementation (for example, an IO action in GHC is a partially applied function, but this is an implementation detail that does nothing to help you understand how to work with it).