Maybe and total interfaces

The idea

Enter with algebraic patterns and polymorphic type variables understood. Maybe a represents either Nothing or Just containing a value of type a. It allows a function to describe absence explicitly without inventing a special element value. A safe first-element function can therefore accept an empty list while retaining the original element type for successful results.

Pattern matching forces the consumer to decide what absence means at its boundary. It might display a message, supply a documented default, or continue with another optional computation. fmap applies a function to a present value while preserving Nothing. Avoid replacing a meaningful distinction too early: Nothing and Just 0 can carry different information even when a later display chooses the same number for both.

safeFirst :: [a] -> Maybe a
safeFirst [] = Nothing
safeFirst (x:_) = Just x
main = print (fmap (+2) (safeFirst [5,9]))

Just 7

safeFirst returns Just 5. fmap transforms its payload by adding two and keeps the result inside Just.