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decimal_checked Tutorial

DecimalChecked is a closed IEEE decimal pipeline. It stores the current defined Decimal, one IEEE DecimalContext, flags raised by the latest step, flags accumulated across all steps, and an optional certification error.

Accumulate IEEE Flags

///|
test "IEEE checked decimal pipeline" {
  let context = @decimal.DecimalContext::new(precision=3)
  let checked = @decimal_checked.DecimalChecked::parse("1.2345", context)
    .add(@decimal.Decimal::one())
  inspect(checked.value().to_string(), content="2.23")
  inspect(
    checked.raised().contains(@decimal.DecimalSignal::Inexact),
    content="true",
  )
  inspect(
    checked.flags().contains(@decimal.DecimalSignal::Rounded),
    content="true",
  )
}

raised() describes only the last operation. flags() is the combined history. outcome() returns the current value with the combined flags.

Keep Defined Exceptional Results

IEEE conditions usually retain a numerical result:

///|
test "IEEE checked defined result" {
  let checked = @decimal_checked.DecimalChecked::from_int(
    1,
    @decimal.DecimalContext::decimal64(),
  ).div(@decimal.Decimal::zero())
  inspect(checked.value().is_infinite(), content="true")
  inspect(
    checked.raised().contains(@decimal.DecimalSignal::DivisionByZero),
    content="true",
  )
  inspect(checked.is_ok(), content="true")
}

is_ok() means that no ArithmeticError stopped the pipeline. It does not mean that IEEE flags are empty. Certification failures from elementary try_* operations enter error() and short-circuit later steps; rounded, inexact, overflow, or division-by-zero conditions remain value-plus-flags outcomes.

Reset A Status Window

clear_flags() keeps the current value and context but clears both latest and accumulated flags. Use it after an application has recorded one calculation phase.

with_context(new_context) reapplies the current value under the new IEEE context and records any resulting flags. It is an observable calculation step, not a metadata-only setter.

Do Not Merge Independent Pipelines

Binary methods accept a plain Decimal. This avoids inventing a rule for merging two contexts and two flag histories.

let right = @decimal.Decimal::from_string("2.5").unwrap()
let result = checked.mul(right)

If two independent checked calculations must be combined, first decide at the application boundary which context and flag-merging policy is correct.

Recommended Practice

  1. Inspect raised() for step-local policy and flags() for end-to-end policy.
  2. Treat is_ok() and flags().has_error() as different questions.
  3. Clear flags only after the previous status window has been consumed.
  4. Use result() at the outer boundary when certification failure must be returned to the caller.
  5. Use decimal_gda_checked instead when sticky GDA status and traps are required.

See Design for the transition model and decimal tutorial for raw context operations.