Haskell Quick Syntax Reference by Stefania Loredana Nita & Marius Mihailescu

Author:Stefania Loredana Nita & Marius Mihailescu
Language: eng
Format: epub
ISBN: 9781484245071
Publisher: Apress

The Three Rules

You have seen that a monad must follow three rules: right unit, left unit, and associativity. These three rules show you the relation between a computation, the return function, and the binding operation.

All monads are instances of the Monad type class from Prelude, which is defined as follows:class Monad m where

return :: a -> m a

fail :: String -> m a

(>>=) :: m a -> (a -> m b) -> m b

(>>) :: m a -> m b -> m b

In this definition, the last rule can be expressed in terms of the third rule. Note that (>>=) is read as “bind,” and (>>) is read as “then.”

Before going further, we need to mention that the do notation works great with monads; it acts as syntactic sugar for operations. You will see some examples in the next sections.

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