LANGUAGE.md - AiToEarn

LANGUAGE.md

1	# Language
2
3	Shared vocabulary for every suggestion this skill makes. Use these terms exactly — don't substitute "component," "service," "API," or "boundary." Consistent language is the whole point.
4
5	## Terms
6
7	Module
8	Anything with an interface and an implementation. Deliberately scale-agnostic — applies equally to a function, class, package, or tier-spanning slice.
9	_Avoid_: unit, component, service.
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11	Interface
12	Everything a caller must know to use the module correctly. Includes the type signature, but also invariants, ordering constraints, error modes, required configuration, and performance characteristics.
13	_Avoid_: API, signature (too narrow — those refer only to the type-level surface).
14
15	Implementation
16	What's inside a module — its body of code. Distinct from Adapter: a thing can be a small adapter with a large implementation (a Postgres repo) or a large adapter with a small implementation (an in-memory fake). Reach for "adapter" when the seam is the topic; "implementation" otherwise.
17
18	Depth
19	Leverage at the interface — the amount of behaviour a caller (or test) can exercise per unit of interface they have to learn. A module is deep when a large amount of behaviour sits behind a small interface. A module is shallow when the interface is nearly as complex as the implementation.
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21	Seam _(from Michael Feathers)_
22	A place where you can alter behaviour without editing in that place. The location at which a module's interface lives. Choosing where to put the seam is its own design decision, distinct from what goes behind it.
23	_Avoid_: boundary (overloaded with DDD's bounded context).
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25	Adapter
26	A concrete thing that satisfies an interface at a seam. Describes role (what slot it fills), not substance (what's inside).
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28	Leverage
29	What callers get from depth. More capability per unit of interface they have to learn. One implementation pays back across N call sites and M tests.
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31	Locality
32	What maintainers get from depth. Change, bugs, knowledge, and verification concentrate at one place rather than spreading across callers. Fix once, fixed everywhere.
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34	## Principles
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36	- Depth is a property of the interface, not the implementation. A deep module can be internally composed of small, mockable, swappable parts — they just aren't part of the interface. A module can have internal seams (private to its implementation, used by its own tests) as well as the external seam at its interface.
37	- The deletion test. Imagine deleting the module. If complexity vanishes, the module wasn't hiding anything (it was a pass-through). If complexity reappears across N callers, the module was earning its keep.
38	- The interface is the test surface. Callers and tests cross the same seam. If you want to test past the interface, the module is probably the wrong shape.
39	- One adapter means a hypothetical seam. Two adapters means a real one. Don't introduce a seam unless something actually varies across it.
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41	## Relationships
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43	- A Module has exactly one Interface (the surface it presents to callers and tests).
44	- Depth is a property of a Module, measured against its Interface.
45	- A Seam is where a Module's Interface lives.
46	- An Adapter sits at a Seam and satisfies the Interface.
47	- Depth produces Leverage for callers and Locality for maintainers.
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49	## Rejected framings
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51	- Depth as ratio of implementation-lines to interface-lines (Ousterhout): rewards padding the implementation. We use depth-as-leverage instead.
52	- "Interface" as the TypeScript `interface` keyword or a class's public methods: too narrow — interface here includes every fact a caller must know.
53	- "Boundary": overloaded with DDD's bounded context. Say seam or interface.
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