# Constructor, explicit, and collection DI

<sub>**Beginner → Intermediate.** Annotations, explicit targets, collections.</sub>

Annotate a constructor parameter. Melder builds the graph.

```python
class ReportService:
    def __init__(self, database: Database) -> None:   # ← injected
        self.database = database

book.bind(spell=Database, existence="unique")
book.bind(spell=ReportService, existence="unique")
conduit = book.conjure()

report = conduit.meld("ReportService")
```

When an annotation isn't specific enough — two implementations of one shape, or
a named binding — declare the target with `SpellMap` as the parameter default:

```python
class Consumer:
    def __init__(self, store=md.SpellMap(PrimaryStore)) -> None:
        self.store = store
```

`SpellMap` takes the full address: a concrete spell, a `spellframe`, a
`binding_name`, or a combination. It obeys an **exactly-one law** — a target
that matches zero spells or several fails at `conjure()`, not at runtime.

Ask for a list and get every implementation — the plugin pattern in one
annotation:

```python
class Dispatcher:
    def __init__(self, handlers: list[Handler]) -> None:   # ← all of them
        self.handlers = handlers

book.bind(spell=EmailHandler, existence="unique",
          spellframe=Handler, binding_name="email")
book.bind(spell=SmsHandler, existence="unique",
          spellframe=Handler, binding_name="sms")
```

## Runnable examples

- [Constructor di by annotation](../examples/intermediate/15-constructor-di-by-annotation.md) — Intermediate 15
- [Spellmap defaults](../examples/intermediate/16-spellmap-defaults.md) — Intermediate 16
- [Collection di](../examples/intermediate/17-collection-di.md) — Intermediate 17

[All intermediate examples](../examples/intermediate/index.md) · [Level contents](index.md) · [Full contents](../contents.md)

[Canonical page source](https://github.com/Synaptic724/melder/blob/a30a754d0bb61db0b142936010cdb82cffecec6f/README.md)