levels-of-abstraction

name	levels-of-abstraction
description	Use when discussing levels of abstraction, composed method, SLAP, stepdown rule, or mixed abstraction levels within a function.

Levels of Abstraction

Every function should operate at a single level of abstraction. When a function calls high-level helpers and does low-level work inline, the reader has to constantly shift mental gears. The fix is always the same: give the low-level block a name.

Terms

Composed Method (Kent Beck, Smalltalk Best Practice Patterns, 1997) A public method that reads as an outline of steps. Each step is a call to a private method with a clear name. The public method states what; the private methods handle how.

Single Level of Abstraction — SLA / SLAP (named by Neal Ford, The Productive Programmer, 2008; restated by Robert C. Martin, Clean Code, 2008) All statements in a function belong to the same level of abstraction. If one statement is at a lower level, extract it.

Stepdown Rule (Robert C. Martin, Clean Code) Functions in a file read top-down: each function is followed by the functions one level of abstraction below it, like a newspaper article narrowing from headline to detail.

Mental Grouping The cognitive work a reader does to assemble missing abstractions — scanning a block of code and figuring out "this chunk does X." A missing method forces the reader to do the extraction in their head.

Smells

• Helpers plus inline logic — a function calls named methods for some steps, then has a raw block of code for another. The raw block is the missing method.
• Comment, blank line, code block — the comment is the name of the method that should exist. Extract and the comment becomes redundant.
• Fat loop body — loop control is one level; the body is a lower level. Extract the body.
• Policy mixed with mechanism — orchestration ("what to do") interleaved with detail ("how to do it") in the same function.

Strategies

See refactoring — Extract function and Extract class are the primary moves. Extract class applies when extract function would need too many parameters (a sign the parameters want to be fields of a cohesive object).