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+# ADR 0016: Callable Pure Go Helpers From the Bounded Client
+
+Date: 2026-06-21
+
+Status: Accepted
+
+## Context
+
+The escape from the bounded `client {}` language (ADR 0008) is binary today:
+either an expression fits the bounded subset, or the whole component is rewritten
+as a hand-authored Go WASM island (ADR 0003 / ADR 0004). There is no middle
+rung, so any logic the bounded language cannot express — non-trivial list
+filtering/sorting, multi-field derivations, domain calculations — forces a
+disproportionate jump to a full island the author owns end to end.
+
+The bounded language is intentionally small and must stay that way (ADR 0008).
+Widening it to cover every transform would erode the "bounded / Go-owned /
+deterministic" principle and grow the surface the compiler must parse,
+type-check, and explain. We want a ladder, not a cliff:
+
+`bounded` → `bounded + rich expressions (#501/#504)` → `bounded + pure Go helpers`
+→ `full WASM island`.
+
+The hard constraint is ADR-level: **#384 — single-source client semantics.**
+Whatever executes the helper must not reintroduce a second, drifting
+implementation of any behavior. There must remain exactly one source of truth.
+
+This ADR decides the three open questions from issue #519: the helper execution
+model, purity enforcement, and type bridging.
+
+## Decision
+
+Add a `compute`/`computed`-level call to **pure, imported Go functions** from the
+bounded client. The reactive shell stays compiler-owned and bounded; only the
+helper body is author Go.
+
+```gwdk
+client {
+  // ui.FilterRows is an ordinary, pure Go function in a sibling/imported package.
+  computed Visible []Row { return ui.FilterRows(Rows, Query) }
+}
+```
+
+### 1. Execution model: compile the helper to WASM from its Go source
+
+The pure Go helper is **compiled to WebAssembly from the same Go source the
+server uses** and invoked through the existing component WASM island ABI (ADR
+0004). It is never transpiled or re-expressed in JavaScript.
+
+This is the only model that satisfies #384 by construction: the helper has one
+implementation (the Go source), and WASM is merely its compiled form. A Go→JS
+transpiler (rejected below) would be a second implementation of Go semantics and
+is exactly the divergence #384 forbids.
+
+Concretely:
+
+- A component that calls a pure Go helper resolves to the **WASM island lane**.
+  Its bounded reactive shell is still generated by the compiler from the bounded
+  client language (the author does not hand-write the island); the pure helpers
+  are linked into the same module and called by the generated shell.
+- The shell ↔ helper boundary reuses ADR 0004's JSON bootstrap/patch ABI and the
+  single expression/runtime spec (the `RuntimeExpressionSpec` that already drives
+  Go/JS parity for builtins). The bounded shell semantics keep their single
+  source; the helper keeps its single source. Neither is duplicated.
+- This is implemented in phases (see Follow-Up) so the first slice is a small,
+  reviewable delta on top of the existing WASM ABI rather than a new runtime.
+
+Two surface details this ADR commits to so implementers do not have to
+re-decide them:
+
+- **Qualified-call syntax.** The bounded client grammar currently only parses a
+  call whose callee is a bare identifier (a component-local helper). A
+  package-qualified helper call such as `ui.FilterRows(...)` requires extending
+  the call grammar to accept a qualified callee `alias.Name`, resolved through
+  the page/component's declared `import` alias to a Go package symbol — the same
+  alias resolution `build {}` already uses for imported build-data functions.
+  Until that grammar extension lands, the example above does not parse; defining
+  it is part of the implementing change.
+- **Package build isolation.** Go compiles every non-build-tag-excluded file in a
+  package for `GOOS=js`, so pointing a helper at a package that also holds
+  server-only files/imports would fail the WASM build even when the helper's own
+  call graph is pure. The helper module is therefore built from an extracted
+  dependency set — only the helper and its transitively pure dependencies — not by
+  compiling the whole sibling package; alternatively the author keeps helpers in a
+  browser-buildable package. The compiler owns the extraction so "an ordinary
+  sibling/imported function" stays usable for common app packages.
+
+### 2. Purity enforcement is mandatory and static
+
+A function callable from the client must be **pure and deterministic**. The
+compiler runs a purity analysis on the helper's call graph and rejects, with
+diagnostics, any function that:
+
+- performs I/O (network, filesystem, `os`, `syscall`, logging);
+- reads or writes package-level mutable state, or captures mutable globals;
+- starts goroutines, uses channels, `sync`, or other concurrency;
+- uses non-determinism (`time.Now`, `math/rand`, map-iteration-order-dependent
+  output, `unsafe`, pointers escaping into observable identity);
+- **mutates a bridged input.** A `computed` helper must not write through its
+  parameters (or values reachable from them); evaluating a derived value may not
+  change its inputs or depend on aliasing. So an in-place mutator like
+  `sort.Slice(rows, …)` on the `Rows` argument is rejected even though `sort` is
+  otherwise pure;
+- calls another function that is not itself pure.
+
+Allowed: parameters (read-only), locals, pure arithmetic/string/slice/struct/map
+value construction, and calls to other functions proven pure. The curated pure
+standard-library allowlist (`strings`, `math`, `strconv`, the non-mutating parts
+of `sort` such as `sort.SliceStable` on a locally-allocated copy, etc.) admits a
+package only for its non-mutating, deterministic surface — a package is not
+blanket-trusted, and its in-place mutators are allowed only on compiler-proven
+local copies. Purity is a transitive property checked over the reachable call
+graph; the result is cached per function. A helper that cannot be proven pure is
+a compile error that names the offending call site, not a silent downgrade.
+
+### 3. Type bridging reuses the bounded-language value model
+
+Helper parameters and results must be **bridgeable types**: the same JSON-encoded
+value model the bounded client already uses for `state`, `props`, and the island
+ABI — scalars (`string`, `bool`, integer/float numerics), structs of bridgeable
+fields, and slices/maps of bridgeable values. The compiler:
+
+- validates each helper argument's bounded-client type against the Go parameter
+  type, and the Go result type against the `computed` declared type;
+- generates the marshal/unmarshal glue at the ABI boundary from the resolved Go
+  types (reusing the typed-result/struct-field machinery already used for SSR
+  load results and contracts);
+- **exposes resolved result-field metadata with the computed value.** A
+  `computed Visible []Row` must publish `Visible[].Name` (and the rest of `Row`'s
+  bridgeable fields) into the client symbol table, not just the top-level array
+  type — otherwise `g:for`, `g:key`, and row interpolations over the helper
+  result would see unknown fields. The same struct-field resolution that backs
+  typed SSR load results provides this metadata;
+- **constrains wide integers.** `int64`/`uint64` values outside the IEEE-754
+  safe-integer range (±(2^53−1)) cannot survive the JSON/number bridge exactly,
+  which would let a correct Go/WASM helper return a value the bounded client
+  observes rounded — breaking single-source semantics. Bridgeable integers are
+  therefore range-checked (or must be encoded as strings) with a diagnostic when a
+  signature can carry out-of-range values. This matches the safe-integer bound the
+  formatting/date builtins already enforce;
+- rejects unbridgeable signatures (channels, funcs, interfaces without a
+  bridgeable concrete contract, `unsafe.Pointer`) with a diagnostic.
+
+No new value kinds are introduced; the bridge is the existing JSON contract.
+
+## Consequences
+
+### Positive
+
+- Turns the bounded→WASM cliff into a ladder: authors reach for a pure Go helper
+  for the one transform that doesn't fit, instead of rewriting the screen.
+- Single source of truth is preserved by construction (#384): the helper is
+  compiled, never re-implemented.
+- Reuses existing infrastructure: the WASM island ABI (0004), the expression
+  runtime spec, and the typed struct-field/result machinery — minimal new
+  surface.
+- Keeps the bounded language small (0008): pressure to add every transform as a
+  builtin is relieved by "extract a pure Go helper."
+
+### Negative
+
+- A component that uses helpers pays the WASM island cost (module size, load) it
+  would not pay as a pure-JS island. The ladder makes this an explicit author
+  choice, but it is a real cost.
+- Purity analysis is non-trivial and must be conservative (false negatives that
+  reject a pure function are acceptable; false positives that admit an impure one
+  are not).
+- Determinism across Go/WASM is inherited from the Go toolchain, but
+  floating-point and map-ordering pitfalls still need test coverage.
+
+### Neutral
+
+- Default JS islands are unchanged; nothing here alters ADR 0003's default.
+- This is the chosen direction; the implementation is phased and not yet built
+  (see Follow-Up). The bounded language and existing islands behave as before
+  until a phase lands.
+
+## Alternatives Considered
+
+- **Transpile the pure Go helper to JavaScript.** Rejected: a Go→JS compiler is a
+  second implementation of Go semantics and is precisely the divergence #384
+  exists to prevent. Even a "subset" transpiler drifts.
+- **Widen the bounded client language to cover the transforms.** Rejected: it
+  grows the surface 0008 deliberately bounds, and never covers the long tail;
+  authors still hit a wall, just later.
+- **Force a full hand-authored WASM island (status quo).** Rejected: that is the
+  cliff this ADR removes; it makes the author own the reactive shell, props,
+  events, and lifecycle for what should be a one-function escape.
+- **A JS island that calls a small WASM "helper sidecar."** Considered as a
+  size-optimization phase (keep the shell in JS, ship only the helper as WASM).
+  Deferred behind the single-module model because two runtimes per page is more
+  moving parts; revisited in Follow-Up if WASM shell size proves prohibitive.
+
+## Follow-Up
+
+- Depends on **#384** (single-source client semantics) landing first, so the
+  bounded shell has one IR/evaluator before helpers cross the boundary.
+- **Phase 1:** allow pure Go helper calls only inside components already in the
+  WASM lane, with purity validation, type bridging, and generated glue. Smallest
+  reviewable delta; no lane change. Note that ADR 0004's component-level `wasm`
+  declaration today points at a *hand-authored* browser Go package that owns the
+  `GOWDKMount/Handle/Destroy` exports. Helper support is a **distinct
+  generated-helper path**: the compiler still owns the reactive-shell exports and
+  links the pure helpers as ordinary called functions (not lifecycle exports), so
+  there is one lifecycle owner. Phase 1 therefore targets compiler-generated WASM
+  components, not the hand-authored-island export contract, which is unchanged.
+- **Phase 2:** auto-promote a bounded JS component to the WASM lane when it calls
+  a helper, so authors do not manage the lane manually.
+- **Phase 3:** evaluate the JS-shell + WASM-helper-sidecar option if WASM shell
+  size is a problem in practice.
+- Each phase ships with purity/bridging diagnostics, a native example, and
+  generated-output tests, per the #519 checklist.
diff --git a/docs/engineering/decisions/README.md b/docs/engineering/decisions/README.md
index 298b0e5a..6724e985 100644
--- a/docs/engineering/decisions/README.md
+++ b/docs/engineering/decisions/README.md
@@ -32,3 +32,10 @@ Recommended naming:
   query-owned elements as the first realtime reactivity source contract.
 - `0013-built-in-tracing-observability.md`: accepted dependency-free
   `runtime/trace` primitives before generated app auto-instrumentation.
+- `0014-addon-runtime-config-split.md`: accepted split between addon config
+  packages and request-time runtime helper packages.
+- `0015-generated-binary-lifecycle-services.md`: accepted generated binary
+  lifecycle service contracts.
+- `0016-pure-go-helpers-from-bounded-client.md`: accepted direction to call pure
+  Go helpers from the bounded client by compiling them to WASM (single-source
+  semantics, purity validation, JSON type bridging).