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Principles

Pure Functions: Push I/O to the Edges

A pure function gives the same answer for the same inputs and changes nothing outside itself. These lessons take a late fee job that reads the clock, reads the database, writes fees and sends mail in one loop, pull the fee rule out into pure functions, and leave the input and output in a short shell at the edge, so Finance's fee cap and its test touch only the rule.

Pick your language

Pure functions: push I/O to the edges in Java, step 2: Cap the fee at 50 eurosJava · ~5 minPure Functions in JavaPure functions: push I/O to the edges in Kotlin, step 2: Cap the fee at 50 eurosKotlin · ~5 minPure Functions in KotlinPure functions: push I/O to the edges in C#, step 2: Cap the fee at 50 eurosC# · ~6 minPure Functions in C#Pure functions: push I/O to the edges in C++, step 2: Cap the fee at 50 eurosC++ · ~6 minPure Functions in C++Pure functions: push I/O to the edges in Python, step 2: Cap the fee at 50 eurosPython · ~5 minPure Functions in PythonPure functions: push I/O to the edges in Ruby, step 2: Cap the fee at 50 eurosRuby · ~6 minPure Functions in RubyPure functions: push I/O to the edges in PHP, step 2: Cap the fee at 50 eurosPHP · ~5 minPure Functions in PHPPure functions: push I/O to the edges in TypeScript, step 2: Cap the fee at 50 eurosTypeScript · ~5 minPure Functions in TypeScriptPure functions: push I/O to the edges in Go, step 2: Cap the fee at 50 eurosGo · ~5 minPure Functions in GoPure functions: push I/O to the edges in Rust, step 2: Cap the fee at 50 eurosRust · ~6 minPure Functions in RustPure functions: push I/O to the edges in Swift, step 2: Cap the fee at 50 eurosSwift · ~5 minPure Functions in Swift

When to use it

Use it when

A business rule sits inside code that talks to a database, the clock, files or the network, and you want to test the rule alone.

Skip it when

The code is only glue with no decision in it, like copying a file and logging that it was copied. There's no rule to pull out.

Spot it by

A now() or a database call in the middle of a calculation, or a test that needs a fake for every collaborator to check one number.