# Folds A **fold** or a **folding function** is a higher-order function that processes a data structure (e.g. a list) in some order and builds a return value along the way. Therefore, a fold needs three things to work - **a function that combines the elements, a starting value, and the data structure** where the elements are stored. There are two basic folding functions defined in the Haskell Prelude, `foldr` and `foldl`. The difference between them is the order in which they apply the combining function to the elements. --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://haskell.hpmeducation.com/recursion/folds.md?ask= ``` The question should be specific, self-contained, and written in natural language. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.