Dive Deep into Higher-Order Functions

A Higher-Order Function takes one or more functions as arguments, returns a function as its result, or both. The article ties this to functions being first-class citizens in functional languages, assignable to variables and passed around like any other value.

The three everyday examples are map (transform each item), filter (select items by a condition) and reduce (combine items into one result). The article calls these the backbone of collection processing across JavaScript, Python, Java, Kotlin, Scala and Haskell. Function composition and currying build on the same base, chaining functions together or turning a multi-argument function into a sequence of single-argument ones.

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• HOFs abstract away repetitive patterns, shifting code from describing how to do something to describing what needs to be done.
• The piece links twenty external resources spanning Medium, Kotlin’s own docs, university lecture PDFs (Utrecht, CMU), FreeCodeCamp, O’Reilly and language-specific guides for JavaScript, Java, C#, Python and Go.
• The recommended starting point for beginners is map/filter/reduce before moving to composition and currying.