fmap
Haskell-function på Functor-typeclass. Apply function till value(s) inside container utan att unwrap. fmap (+1) (Just 5) = Just 6. JavaScript-equivalent: [1,2,3].map(x => x+1).
Type-signature: fmap :: Functor f => (a -> b) -> f a -> f b. Generic över container-type — Maybe, List, Either, IO. Operator-form: <$>. (+1) <$> [1,2,3]. Modern equivalent i Rust: Option::map, Result::map, Iterator::map — alla samma pattern. TypeScript: Array.prototype.map, Promise.then. fmap-laws: identity (fmap id = id), composition (fmap (f . g) = fmap f . fmap g). Foundation för Functor + later Applicative + Monad. Modern FP-pattern: build everything via fmap + chain.