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Varianz stages are written in CEL (Common Expression Language) — sandboxed, deterministic, and safe to push to running services. Expressions compile against the target VPoint’s schema when inserted, so type and field errors surface at insert time.

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Namespaces

args — the VPoint’s inputs. Parameters are reachable only through args:
Parameter names follow the SDK’s schema naming (for example Go’s BasePrice is args.base_price — see each SDK’s field-naming rules). ctx — context functions registered by the service: ctx.applyTax(invoke(), 0.08). See context functions.

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Varianz builtins

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invoke()

Executes the next stage in the pipeline — or, at the end of the chain, the real function — and returns its result.
  • invoke() with zero arguments forwards the original arguments.
  • invoke(a, b, ...) with exactly the VPoint’s parameter count substitutes different arguments.
  • Any other arity is a compile error.
An expression without invoke() fully replaces the function’s result; the real code never runs.

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sample(name, value)

Captures value under the label name for the test session to assert on, and returns value unchanged. Exactly two arguments. Because it’s transparent, it composes anywhere:

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Struct construction

  • Type names must match the schema exactly and are case-sensitive. Use the simple name (ChargeResult), never a package-qualified one (pb.ChargeResult, hipstershop.ChargeResult).
  • All required fields must be provided.
  • Nested structs work inline: GetProductOutput { id: "X", price: Money { currency_code: "USD", units: 42, nanos: 0 } }.

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Supported syntax

Tuple values expose synthetic positional fields: read pair._0 (or pair[0] when the schema proves a fixed tuple), construct with Pair { _0: "north", _1: 8 }. Generated wrapper names may be lowercase.

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Optionals and null at the stage boundary

During evaluation, CEL keeps the standard distinction — optional.of(null).hasValue() is true, optional.none().hasValue() is false. At the stage boundary, however, Varianz transports an optional as either its inner value or absent: both optional.of(null) and optional.none() materialize as absence to the SDK on the other side.

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Not yet supported

  • Comprehensions map, filter, exists_one (only exists/all)
  • Type conversions (int(), string(), …) and date/time functions
  • Error diagnostics carry byte offsets, not line/column

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Where CEL runs

CEL stages attach to a VPoint at an anchor — MID by default — scoped to your session. Insert them from any test SDK (cel(target, expression)); target syntax and routing filters are in Naming and routing.