pippinmole/ghost

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Source code of Ghost - On Ghost

chore: add the code-review and cleanup skills used for the unslop pass6b064615 days ago114 lines · 6.3 KB

1	---
2	name: codebase-design
3	description: Shared vocabulary for designing deep modules. Use when the user wants to design or improve a module's interface, find deepening opportunities, decide where a seam goes, make code more testable or AI-navigable, or when another skill needs the deep-module vocabulary.
4	---
5
6	# Codebase Design
7
8	Design deep modules: a lot of behaviour behind a small interface, placed at a clean seam, testable through that interface. Use this language and these principles wherever code is being designed or restructured. The aim is leverage for callers, locality for maintainers, and testability for everyone.
9
10	## Glossary
11
12	Use these terms exactly: don't substitute "component," "service," "API," or "boundary." Consistent language is the whole point.
13
14	Module: anything with an interface and an implementation. Deliberately scale-agnostic: a function, class, package, or tier-spanning slice. _
15
16	Interface: everything a caller must know to use the module correctly: the type signature, but also invariants, ordering constraints, error modes, required configuration, and performance characteristics. _
17
18	Implementation: what's inside a module, its body of code. Distinct from **
19
20	Depth: 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 **
21
22	Seam _
23
24	Adapter: a concrete thing that satisfies an interface at a seam. Describes *
25
26	Leverage: what callers get from depth. More capability per unit of interface they learn. One implementation pays back across N call sites and M tests.
27
28	Locality: what maintainers get from depth. Change, bugs, knowledge, and verification concentrate in one place rather than spreading across callers. Fix once, fixed everywhere.
29
30	## Deep vs shallow
31
32	Deep module = small interface + lots of implementation:
33
34	```
35	┌─────────────────────┐
36	│ Small Interface │ ← Few methods, simple params
37	├─────────────────────┤
38	│ │
39	│ Deep Implementation│ ← Complex logic hidden
40	│ │
41	└─────────────────────┘
42	```
43
44	Shallow module = large interface + little implementation (avoid):
45
46	```
47	┌─────────────────────────────────┐
48	│ Large Interface │ ← Many methods, complex params
49	├─────────────────────────────────┤
50	│ Thin Implementation │ ← Just passes through
51	└─────────────────────────────────┘
52	```
53
54	When designing an interface, ask:
55
56	- Can I reduce the number of methods?
57	- Can I simplify the parameters?
58	- Can I hide more complexity inside?
59
60	## Principles
61
62	- 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
63	- The deletion test. Imagine deleting the module. If complexity vanishes, it was a pass-through. If complexity reappears across N callers, it was earning its keep.
64	- The interface is the test surface. Callers and tests cross the same seam. If you want to test
65	- One adapter means a hypothetical seam. Two adapters means a real one. Don't introduce a seam unless something actually varies across it.
66
67	## Designing for testability
68
69	Good interfaces make testing natural:
70
71	1. Accept dependencies, don't create them.
72
73	```typescript
74	// Testable
75	function processOrder(order
76
77	// Hard to test
78	function processOrder(order
79	const gateway =
80	}
81	```
82
83	2. Return results, don't produce side effects.
84
85	```typescript
86	// Testable
87	function calculateDiscount(cart
88
89	// Hard to test
90	function applyDiscount(cart
91	cart.total -=
92	}
93	```
94
95	3. Small surface area. Fewer methods = fewer tests needed. Fewer params = simpler test setup.
96
97	## Relationships
98
99	- A Module has exactly one
100	- Depth is a property of a
101	- A Seam is where a
102	- An Adapter sits at a
103	- Depth produces
104
105	## Rejected framings
106
107	- Depth as ratio of implementation-lines to interface-lines (Ousterhout): rewards padding the implementation. We use depth-as-leverage instead.
108	- **"Interface" as the TypeScript `interface
109	- "Boundary": overloaded with DDD's bounded context. Say
110
111	## Going deeper
112
113	- Deepening a cluster given its dependencies, see
114	- Exploring alternative interfaces, see