Closures and Higher-Order Functions

Closures and Higher-Order Functions

In this lesson — part of Advanced Features — you'll learn closures and higher-order functions in Haskell and why it matters in real work.

Why it matters

Functions that take or return functions unlock concise, composable code.

Key ideas

• Functions as values
• map / filter / reduce
• Callbacks
• Composition

In practice

Here's how it looks in idiomatic Haskell:

-- Functions taking/returning functions are everyday tools
applyTwice :: (a -> a) -> a -> a
applyTwice f = f . f          -- (.) composes two functions

result :: [Int]
result = map (applyTwice (+ 3)) [1, 2, 3]   -- [7, 8, 9]

Haskell note: Composition (.) and application ($) are the glue of point-free style, letting you build pipelines like f . g . h with no named intermediate arguments.

Try it yourself

Exercise: In Haskell, use map and filter to get the squares of the even numbers.

Recap

You now understand closures and higher-order functions and can apply it in Haskell. Mark this lesson complete and continue to the next one.