A monad sequences computations while carrying a context such as optionality, state, errors, input-output operations, or other effects. It separates data from the processes that compute with it. section 1.5.4
Consider retrieving a user, reading the user’s name, and taking its first letter. Each lookup may fail, so every step returns a Maybe value. Nested case expressions correctly propagate Nothing, but the surrounding plumbing obscures the computation’s intent. section 1.5.4
getUserInitials :: Int -> Maybe Char
getUserInitials key =
case getUser key users of #1
Nothing -> Nothing
Just user -> case getUserName user of #2
Nothing -> Nothing
Just name -> Just (head name)
The explicit version handles each possible failure with nested case expressions before producing the user’s initial. section 1.5.4
getUserInitials' :: Int -> Maybe Char getUserInitials' key = do user <- getUser key users #1 name <- getUserName user #2 Just (head name)
The monadic version expresses the same sequence directly: obtain a user, obtain a name, and construct the result while Maybe propagates failure. section 1.5.4
Monads support several design and implementation roles:
- Encapsulate effects such as state changes, errors, and input-output operations. section 1.5.4
- Reduce accidental complexity in functional code, including callback-heavy workflows and parser construction. section 1.5.3
- Provide a functional interface through an algebraic structure whose operations and invariants can be reused by libraries. section 1.5.4 section 1.3.3
From optional lookups to monadic scripting
The Maybe example sequences database-style lookups without repeating failure branches. At a larger architectural scale, free monadic eDSLs use monadic scripting to express business logic and support interpretable domain-specific languages. section 1.5.4 section 6.4
