📘
HPM Education - Haskell
  • Introduction to Haskell
  • Introduction
    • Functions
    • Functional Programming vs Imperative Programming
    • Installing Haskell
    • Haskell Modules
    • Loading Modules into GHCi
    • Expressions
    • Laziness
    • Immutability
  • Types in Haskell
    • Introduction
    • Basic Types
    • Static Type Check
    • Polymorphic and Overloaded Types
    • Data Structure Types
      • Lists
        • List Functions
      • Tuples
    • Function Types
      • Curried Functions
      • Partial Application
  • Defining Functions / Working with Functions
    • The Layout Rule
    • Local Definitions
    • The Infix Operator
    • Conditionals
      • If-then-else Statements
      • MultiWayIf
      • Guarded Equations
      • Case-of Statements
    • Pattern Matching
      • Tuple Patterns
      • List Patterns
    • Lambda functions
    • Function Operators
  • List Comprehensions
    • List Comprehensions
  • Higher-order Functions
    • Introduction
    • The map Function
    • The filter Function
  • Recursion
    • Introduction
    • 4 Steps to Defining Recursive Functions
    • Recursion Practice
    • Folds
      • Fold Right (foldr)
      • Fold Left (foldl)
  • Cutom Types
    • Declaring Types
      • Type Synonyms
      • Data Declarations
      • Newtype declarations
  • Type Classes
    • Introduction
    • Basic Classes
      • Eq – Equality Types
      • Ord – ordered types
      • Show – Showable Types
      • Read – readable types
      • Num – Numeric Types
      • Integral – Integral Types
      • Fractional – Fractional Types
      • Enum – Enumeration Types
    • Derived Instances
    • Exercise – Making a Card Deck Type
  • Interactive Programming
    • Introduction
    • Input / Output Actions
    • Sequencing Actions
    • Exercise - Numbers Guessing Game
  • Functors, Applicatives and Monads
    • Introduction
    • Functors
    • Applicative Functors
    • Monads
      • Maybe Monad
      • List Monad
      • Monad Laws
  • References / Further Reading
Powered by GitBook
On this page

Was this helpful?

  1. Higher-order Functions

Introduction

PreviousList ComprehensionsNextThe map Function

Last updated 2 years ago

Was this helpful?

A higher-order function in Haskell is a function that either takes in a function as its argument or returns a function as its result. We have already seen how functions can return other functions as their result when we introduced , so we will focus on functions that take other functions as arguments in this chapter. First, let's look at a simple example of a higher-order function that takes in a function and applies it twice to an argument:

applyTwice :: (a -> a) -> a -> a
applyTwice f x = f (f x)

ghci> applyTwice (++ " two") "one"
"one two two"

The function we passed in (++ "two ") simply appends the string "two " to the argument passed in (in this case, it must be a string), and it is applied twice when the higher-order function applyTwice is called.

Now let's take a closer look at two higher-order functions that are defined in the Prelude for working with lists, map and filter.

curried functions