> For the complete documentation index, see [llms.txt](https://as-pect.gitbook.io/as-pect/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://as-pect.gitbook.io/as-pect/.pi/skills/tdd/deep-modules.md).

# Deep Modules

From "A Philosophy of Software Design":

**Deep module** = small interface + lots of implementation

```
┌─────────────────────┐
│   Small Interface   │  ← Few methods, simple params
├─────────────────────┤
│                     │
│                     │
│  Deep Implementation│  ← Complex logic hidden
│                     │
│                     │
└─────────────────────┘
```

**Shallow module** = large interface + little implementation (avoid)

```
┌─────────────────────────────────┐
│       Large Interface           │  ← Many methods, complex params
├─────────────────────────────────┤
│  Thin Implementation            │  ← Just passes through
└─────────────────────────────────┘
```

When designing interfaces, ask:

* Can I reduce the number of methods?
* Can I simplify the parameters?
* Can I hide more complexity inside?