---
myst:
  html_meta:
    description: "Learn constructor dependency injection in Melder and follow a complete runnable Python example with typed dependencies."
---

# Constructor dependency injection

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

Annotate a constructor parameter. Melder builds the graph.

What an annotation selects depends on what it is. A **class** selects the spells of that type — a class
bound bare, or an existing object of that class — and nothing else: a category that happens to share the
class's name does not turn its members into providers. A **Protocol** selects the spells bound under it as
their contract. A **`TYPE_CHECKING`-only import**, which leaves a string at runtime, resolves by name to a
type or a contract, never to a string category. `list[...]` gathers the group the annotation's kind names:
every spell of the class, every implementer of the Protocol, or — for a string — every member of the
category or type of that name.

```python
import melder as md


class Database:
    """A small stand-in for a database; this example performs no I/O."""

    def query(self) -> str:
        return "42 rows"


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

    def run(self) -> str:
        return "report over " + self.database.query()


book = md.Spellbook()
book.bind(spell=Database, existence="unique")
book.bind(spell=ReportService, existence="unique")

with book.conjure() as conduit:
    report: ReportService = conduit.meld("ReportService")
    assert isinstance(report.database, Database)
    print(report.run())
```

This complete example prints `report over 42 rows`: melding the consumer first
constructs its registered dependency and injects it into the constructor.

## Runnable examples

- [Constructor di by annotation](../examples/intermediate/15-constructor-di-by-annotation.md) — Intermediate 15
- [Typed melding](../examples/beginner/16-typed-melding.md) — Beginner 16

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

[Canonical page source](https://github.com/Synaptic724/melder/blob/90d52e0a12cb0743d62568d5a19ce954a54443bc/README.md)