[New Concept Docs]: Higher Order Functions

[BethanyG]

This issue describes how to implement the higher-order functions in Python concept docs. You can find the related concept exercise issue here

If you have not yet contributed to concept documents, this issue will require some upfront reading to give you the needed background knowledge.

✅ Getting started

Please please read the docs before starting. Posting PRs without reading these docs will be a lot more frustrating for you during the review cycle, and exhaust Exercism's maintainers' time.

General Contributing Docs:

• Contributing to Exercism | Exercism and GitHub | - Contributor Pull Request Guide
• What are those Weird Task Tags about?
• Exercism Formatting and Style Guide
• Exercism Markdown Specification
• Reputation

Documents on Language Tracks and Concepts:

• Building Language Tracks: An Overview
• What are Concepts?
• Concept Specifications

🎯 Goal

These concept docs are meant to teach a deeper understanding/use/creation of higher-order functions in Python.

💡 Learning objectives

• Understand and create higher-order functions in Python
• Create functions used as arguments to other functions
• Create functions that return functions
• Learn about currying
• Understand and create closures -- where an inner function has access to a variable from it's enclosing scope (the outer function) that has completed its execution.
• Understand and use a simple recursion-- where a function calls itself as an argument.

🚫 Out of scope

Concepts & Subjects that are Out of Scope (click to open)

This exercise assumes an understanding of its prerequisites, so it is not necessary to go into scoping, function special attributes, function arguments, or other concepts "higher" in the syllabus "tree".

• comprehensions
• decorators (these will have their own exercise)
• functools (this will get its own exercise)
• functools.wraps
• generators
• lambda, anonymous functions
• map(), filter(), and reduce() (these will get their own exercise)
• nonlocal & locals()
• global & globals()
• argument unpacking

🤔 Concepts

Concepts and Related Concepts this Concept Covers (click to open)

• closures
• currying
• functions, higher-order functions
• functions as arguments
• functions as returns
• recursion

↩️ Prerequisites

These are the concepts/concept exercises the student should be familiar with before taking on/learning this concept.

Prereqs (click to open)

• basics
• booleans
• comparisons
• dicts
• dict-methods
• functions
• function-arguments
• argument unpacking
• iteration
• lists
• list-methods
• numbers
• sequences
• sets
• strings
• string-methods
• tuples

📚 Resources to refer to

Resources (click to open)

• Python Docs: Python Scopes and Namespaces
• Python Docs: Defining Functions
• Dan Bader: Python s Functions are First-Class
• Functions as Objects in Python
• Composing Programs: Higher-Order Functions
• Tobias Kohn: Closures in Python
• zetcode: Python Closures
• Sagnick: Python is the Haskell you Never Knew You Had: Currying
• Oreilly: Associating Parameters with a Function (Currying)
• Real Python: Thinking Recursively in Python
• Train Your Brain to Think Recursively

📁 Files to Be Created

File Detail for these Concept Docs

Please see the following for more details on these files: concepts links.json For more information, see concept links file The same resources listed in this issue can be used as a starting point for the concepts/links.json file, if it doesn't already exist. If there are particularly good/interesting information sources for this concept that extend or supplement the concept exercise material & the resources already listed -- please add them to the links.json document. Concept about.md For more information, see Concept about.md This file provides information about this concept for a student who has completed the corresponding concept exercise. It is intended as a reference for continued learning. Concept introduction.md For more information, see Concept introduction.md This can also be a summary/paraphrase of the about.md document listed above, and will provide a brief introduction of the concept for a student who has not yet completed the associated concept or practice exercises. It should contain a good summation of the concept, but not go into lots of detail. Concept .meta/config.json Entries For more information, see Concept .meta/config.json This file is likely already stubbed out. Remember to add a concept blurb of less than 350 characters. Please also add your GitHub username to the "authors" array, and any contributor GitHub usernames to the "contributors" array.

🎶 Implementation Notes

• Example code should only use syntax & concepts introduced within these docs or one of the prerequisite concept exercises or documents. Where possible, please use REPL formatting, unless you are demonstrating pseudo code or a long code block. Please do not use syntax not previously covered in prerequisite topics or exercises. Please also follow PEP8 guidelines.
• Our markdown and JSON files are checked against prettier . We recommend setting prettier up locally and running it prior to submitting your PR to avoid any CI errors.

🆘 Next Steps & Getting Help

1. If you'd like to work on this issue, comment saying "I'd like to work on this" (there is no real need to wait for a response, just go ahead, we'll assign you and put a [claimed] label on the issue).
2. If you have any questions while implementing, please post the questions as comments in here, or contact one of the maintainers on our Slack channel.