Default Values

Null Default

In general, the smithy.api#default trait with a null value has no effect. The only exception is when the shape is explicitly annotated as nullable, in which case Smithy4s will then treat Nullable.Null as the default value.

structure Test {
  @default // same thing as @default(null)
  one: String
}

Decoding and defaults

The following table shows different scenarios for decoding of a structure named Foo with a single field s. The type of the field will differ depending on the scenario. However, the input for each scenario is the same: an empty JSON Object ({}). We are using JSON to show this behavior (based on the smithy4s json module), but the same is true of how smithy4s decodes Document with Document.DObject(Map.empty) as an input.

Required	Nullable	Null Default	Scala Representation	Input:
false	true	true	Foo(s: Nullable[String])	Foo(Null)
false	true	false	Foo(s: Option[Nullable[String]])	Foo(None)
false	false	true	Foo(s: Option[String])	Foo(None)
false	false	false	Foo(s: Option[String])	Foo(None)
true	false	false	Foo(s: String)	Missing required field error
true	false	true	Foo(s: String)	Missing required field error
true	true	false	Foo(s: Nullable[String])	Missing required field error
true	true	true	Foo(s: Nullable[String])	Foo(Null)

Key for Table Above

• Required - True if the field is required, false if not (using smithy.api#required trait)
• Nullable - True if the field is nullable, false if not (using alloy#nullable trait)
• Null Default - True if the field has a default value of null, false if it has no default (using smithy.api#default trait)
• Scala Representation - Shows what type is generated for this schema by smithy4s
• Input: - Shows the result of what smithy4s will return when decoding the input of an empty JSON object ({})