Derives: String representation (Reference)

This page documents Debug and Display derives and their behavior.

See also:

• Derives & traits

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Debug (Automatic)

• Format: {value:?}
• User override: not supported
• Intended behavior: structured output (type name + fields)

model Point:
    x: int
    y: int

def main() -> None:
    p = Point(x=10, y=20)
    println(f"{p:?}")  # Point { x: 10, y: 20 }

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Display (Automatic, customizable with __str__)

• Format: {value}
• Default behavior: types have a default Display representation (similar to Python’s default __str__)
• Custom behavior: define __str__(self) -> str
• Conflict rule: if you define __str__, do not also @derive(Display)

model User:
    name: str
    email: str

    def __str__(self) -> str:
        return f"{self.name} <{self.email}>"

def main() -> None:
    u = User(name="Alice", email="alice@example.com")
    println(f"{u}")    # Alice <alice@example.com>
    println(f"{u:?}")  # User { name: "Alice", email: "alice@example.com" }

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Debug vs Display (quick guide)

Aspect	Debug ({:?})	Display ({})
Purpose	developers/logs	users/output
Customizable	no	yes (__str__)