Type Parameter Defaults¶

Generic type parameters can have default types, allowing callers to omit type arguments when the default is appropriate. Inspired by Python PEP 696.

Syntax¶

Use = Type after the type parameter name:

class Box[T = int]:
    value: T

    def __init__(self, value: T):
        self.value = value

    def get(self) -> T:
        return self.value

When using the class, the type argument can be omitted to use the default:

b = Box[int](42)      # Explicit: T = int
s = Box[str]("hello") # Explicit: T = str

Ordering Rule¶

Once one type parameter has a default, all subsequent parameters must also have defaults:

# OK: defaults at the end
class Pair[K, V = str]:
    pass

# OK: all have defaults
class Config[K = str, V = int]:
    pass

# ERROR (SPY0395): non-default follows default
class Bad[T = int, U]:   # U has no default but T does
    pass

Partial Defaults¶

When only some parameters have defaults, parameters without defaults must come first:

class Container[K, V = list[K]]:
    key: K
    value: V

    def __init__(self, key: K, value: V):
        self.key = key
        self.value = value

Constraint Satisfaction¶

Default types must satisfy any constraints on the type parameter. If a default type violates a constraint, the compiler emits SPY0396:

interface IComparable:
    def compare_to(self, other: Self) -> int:
        ...

# ERROR (SPY0396) if default type doesn't satisfy constraint
class Sorted[T: IComparable = int]:
    pass

Diagnostics¶

Code	Level	Description
SPY0395	Error	Type parameter without default follows one with a default
SPY0396	Error	Default type violates type parameter constraint

Generated C¶

Type parameter defaults are resolved at compile time. The generated C# uses concrete type arguments — there is no runtime default mechanism:

class Box[T = int]:
    value: T

generates a generic class Box<T> in C#. The default is used during type inference when the caller omits the type argument.

Implementation - ✅ Implemented — TypeParameterDef.DefaultType property, parsed in Parser.Definitions.cs - Ordering validation: SPY0395 - Constraint satisfaction: SPY0396