Conditional Fields

Conditional Fields with when

Use when blocks to conditionally include fields based on a runtime value:

Record Entry(has_extras) = {
  id: UInt32LE,
  value: Float64LE,
  when has_extras == 1 {
    extra1: UInt32LE,
    extra2: UInt32LE
  }
}

The when block is only executed (and its fields only consumed) if the condition evaluates to a non-zero value.

Block Form

The currently supported syntax uses braces to group conditional fields:

when condition {
  field1: Type1,
  field2: Type2
}

Multiple fields can appear inside a single when block.

Conditions

Conditions can use any combination of variables, parameters, comparisons, and logical operators:

Record FlexibleEntry(version, has_checksum) = {
  id: UInt32LE,
  data: UInt16LE,
  when version >= 2 {
    extended_data: UInt32LE
  }
  when has_checksum {
    checksum: UInt32LE
  }
}

Nesting

when blocks can be nested:

when version >= 2 {
  extended: UInt32LE,
  when has_extras == 1 {
    bonus: UInt16LE
  }
}

Important Restrictions

• Field references in conditions are not supported for member access. You cannot reference fields consumed inside a when block from outside of it.