Monomorphization ABI Guide

This guide explains the codegen ABI that Turmeric settled on after the end-to-end monomorphization migration (concluded 2026-06-19). It is aimed at two audiences:

• Library/spice authors who want to understand why a function's generated C signature looks the way it does, and how that affects inline-C bridging and FFI.
• Compiler contributors who need a working model of the by-value path before touching elaborator/codegen code.

If you only ever read Turmeric source and never write inline-C, you can get away without this guide -- the language surface hides the ABI. You'll start needing it the first time you see a symbol like option_map__spec__int_Option__cstr in a link error.

TL;DR

• A polymorphic Turmeric value uses its natural C layout at every call site (int64_t, double, a defstruct's C struct, an opaque pointer, etc.) -- not a uniform int64_t "carrier".
• The elaborator picks each call site's concrete type and emits a per-call-site specialization named <defn>__spec__<arg-types>.
• Constrained-polymorphic functions (those with typeclass dictionaries) monomorphize per dictionary: one spec per concrete instance.
• A small carrier bridge still exists for two intentional cases (recursive combinator HKT instances whose closures return an HKT-applied type; consumer-side bridges where typing the producer would be churn-for-no-gain). It is documented and load-bearing -- not a migration loose end. As of 0.22.0 (#444) the by-value path's audit floor is 0 carrier deref-copies; the Vec and MutableMap element bridges that previously appeared here have been retired.

Why monomorphization, not a uniform carrier

The pre-2026 ABI threaded every polymorphic value as int64_t -- a "carrier" wide enough to hold any pointer or primitive, with implicit boxing/unboxing at type boundaries. The carrier worked, but each type-system feature shipped that year (HKTs, sized types, substructural caps, refinement types, associated types) ended in an ABI-patch bug report:

• option-none-as-null-byvalue-param-segfault
• defstruct-byvalue-struct-field-stored-as-int-carrier
• tourist-captures-reads-back-as-int-carrier-collapse
• letrec-self-recursive-float-carrier-collapse
• ...and dozens more (see docs/archive/history/).

The pattern: a value would be classified as concrete (Pos, double, Option<int>) in one phase and as a carrier (int64_t) in the next, and a cast was missing at the boundary. Patches stacked up in elaborator and codegen until the cumulative cost motivated picking one ABI: by-value end-to-end.

The win:

• Struct values pass and return as real C structs. No intptr_t cast.
• Floats pass as double. The whole "polymorphic float carrier ascription" class of bugs is structurally impossible.
• Generated C is readable; debugging a codegen issue is a matter of reading the emitted spec, not chasing a cast that should have fired.
• Optimizers (clang -O2, ASan field-precision) see the real types.

The cost: more code generated. A (option-map f xs) used five places in your project, each with different element types, lowers to five specs (option_map__spec__int, option_map__spec__float, option_map__spec__Pos, ...). This is the same trade Rust makes; in practice the cost is fine because (a) most call sites share types, (b) dead-spec elimination prunes unused ones, and (c) the binary stays small because each spec is tiny.

What gets monomorphized

The spec name is deterministic: same call-site types → same spec name → no symbol collisions across separate compilation units. If two modules instantiate option-map with int they refer to the same emitted symbol; only one TU actually emits the body (the rest declare extern).

How to read a spec symbol in an error message

Most ABI-flavored errors look like one of:

undefined reference to `option_map__spec__int_Option__cstr'
multiple definition of `some___spec__bool_Option__opaque'

Parse it:

1. The first segment up to __spec__ is the originating defn/method/ctor.
2. Everything after is the argument-type signature, in elaboration order, with double-underscore separators. Type applications use F__A -- Option__cstr means (Option cstr).

some___spec__bool_Option__opaque = the some constructor specialized for the type (Option (Option <opaque>)) (note the triple underscore distinguishing constructor-name from method-name).

If you see one of these in a link error, the usual causes are:

• Linkage mismatch. A spec was emitted static in one TU and declared extern in another. The fix lives in the compiler -- file a report. The historical example is the prelude-spec linkage story: pre-0.21.0, two TUs referencing the same prelude spec would multiple definition link-fail; 0.21.0 worked around it by emitting prelude specs static in every referencing TU; 0.22.0 (#444 follow-up) flipped them back to external linkage now that the underlying cross-TU ownership rule is correct. If you see this class of error today, it almost always means a spec is being claimed by two owners.
• Missing import. A module uses a spec that no module in the project actually instantiates. Add the import that drives the instantiation, or move the call site into the using module.
• #[used] defn reached only by mangled symbol. A pure-Turmeric defn called only via a hand-written inline-C bridge or a C-ABI callback was DCE'd. Add #[used] to the defn so the compiler keeps external linkage.

The residual carrier bridge -- what it does, why it stays

Phase 5 of the migration set out to delete the carrier bridge entirely. As of 0.22.0 (#444), two classes of crossings remain -- either intentional, or load-bearing in a way that no method-level fix can address:

1. Consumer-side bridges, by design. Some inline-C extractors take an int64_t argument and reinterpret. The audit fixture coverage requires this to stay working so the bridge is regression- tested. Removing it removes the test, not a bug.
2. HOF return-type closures. Monad bind / Applicative ap take a continuation (fn [A] (m B)) whose result is an HKT-applied type. Threading the result by-value end-to-end requires retyping the parser/combinator library (Parser A and its continuations) plus propagating generic-dispatch type information through the entire call graph. That's a parser-rewrite unit of work, not a localized ABI fix. Until it lands, the closure's return value rides the carrier and a tiny spill shim boxes it. 0.22.0 #438 narrowed this considerably -- Applicative ap now preserves fn type through polymorphic constructors -- but the residual continuation case still rides the bridge.

The Vec and MutableMap element bridges that previously sat in this section are gone: 0.21.0 #377 migrated Vec to the :heap typed-pointer ABI (retiring carrier-based Eq[Vec]); 0.22.0 #396 retyped MutableMap to the honest (MutableMap K V) and retired its carrier bridge; 0.22.0 #391/#393 monomorphized the Vec inline-C producers to typed pointers.

The bridge is now a documented small surface, not an open migration. Look for tur_box_T / tur_unbox_T helpers in the runtime and the ensure_aggregate_spill_shim codegen helper for the spill path.

Practical impact on day-to-day Turmeric code

You should mostly not notice the ABI. The places it surfaces:

Inline-C blocks see real C types

A defn with #fx{Unsafe} and an inline-C body sees the parameter types as their natural C layout:

(defn add-pos [a : Pos b : Pos] #{Unsafe} : Pos
  ```c
  Pos out;
  out.x = a.x + b.x;
  out.y = a.y + b.y;
  return out;
  ```)

Before monomorphization landed, you would have had to cast a and b from int64_t back to Pos* and dereference. That style is no longer needed (and is wrong now -- a and b are passed by value).

The carrier-bridge corners (HKT continuation results, Vec/Map element reinterprets) are the only places you still write (int64_t)(intptr_t) casts. If you find yourself reaching for one outside those cases, the typechecker probably wants a real type.

FFI / C-ABI callbacks

When you hand a Turmeric function pointer to a C library (qsort comparators, Arrow release callbacks, signal handlers, raylib draw callbacks), the mangled spec symbol is what the C side takes. Mark the defn #[used] (shipped 0.22.0, #467) so the compiler keeps external linkage and doesn't DCE the body:

(defn #[used] compare-ints [a : ptr<void> b : ptr<void>] : int
  ```c
  int ai = *(const int*)a;
  int bi = *(const int*)b;
  return (ai > bi) - (ai < bi);
  ```)

Without #[used], the symbol is static in the emitted C and the extern C reference fails at link. With it, external linkage is preserved and every project module compiles and links.

Separate compilation

Each module compiles independently. Spec emission rules:

• A spec owned by a real user module is emitted with external linkage in that module and extern declarations elsewhere.
• A spec owned by the prelude (no real owning module) is also emitted with external linkage as of 0.22.0 (#444 follow-up): the prior 0.21.0 workaround that emitted prelude specs static in every referencing TU is gone now that the cross-TU ownership rule is correct. The history: pre-0.21.0, two TUs sharing a prelude spec would multiple definition link-fail; 0.21.0 patched that by going static; 0.22.0 returned them to external linkage with the ownership rule fixed.

If you're maintaining the codegen, the rule of thumb is: the owning module is whoever declared the originating defn/instance. If that module is the prelude, fall back to static.

Compiler-contributor cheat sheet

• Emit pipeline. emit_call.c picks the spec for a call site; emit_fn.c emits each spec's body. mangle.c turns (defn-name, [arg types]) into the spec symbol.
• Boundary types. TypeKind::Carrier (int64_t) and TypeKind::Concrete (the real type) are the only two flavors that matter at the C-emit boundary. 0.21.0 #368 removed the EX_ASCRIBE CK_CONCRETE -> CK_CARRIER bridge; the remaining ascription path goes the other direction (carrier -> concrete return deref for by-value instance methods, landed in 0.21.0 #369 under M5 Option C). Outside the documented bridge cases, every Concrete stays Concrete.
• HKT specs. A by-value HKT instance method's spec name encodes both the constructor and the element type: Functor_fmap__spec__Option__int. If you see a method spec without the trailing element-type segment, that's a carrier-path spec -- legitimate for instances whose body has no by-value rewrite (e.g. Functor[Parser]) but unexpected for Option/Result.
• Construct-recovery. #fx{Construct} ctors (ok, err, some, none) lower to a spec selected from the expected type at the call site, not the type of the argument. This is how (none) knows to emit none__spec__int vs none__spec__cstr. If a (none) call picks the wrong spec, the issue is in result-type propagation, not in the constructor lowering.
• Dead-spec elimination. A spec referenced by no module gets pruned. If a spec mysteriously vanishes, check whether the only references were behind a mangled C symbol (needs #[used]) or were themselves DCE'd.
• Bare generic ADT rides the carrier; only concrete gets a struct. An unparameterized Result/Option with no type args pinned lowers to the int64_t boxed carrier; the by-value aggregate struct (tur_adt_Result__int__cstr) exists only for a concrete monomorphized instantiation. So only concrete instantiations get struct DWARF and struct pretty-printers, and any construct that yields a bare Result (e.g. catch-unwind handing back (Result ThunkRet Panic) with an intentionally-open err arm) returns the carrier. Bridging that carrier into a declared by-value aggregate return needs a structural field-by-field rebuild from the box -- a representational emit_type_c_name(...) == "int64_t" test misfires, because ok/err/some/none constructors also collapse to the int64 carrier under emit_type_c_name yet emit the aggregate directly.
• Aggregate param pass-mode: read the Type, not the C-name. type_c_name yields the bare struct name for both small (by-value) and large (by-ref, const T*) products, so classifying an aggregate param's pass-mode by C-name string alone misclassifies a by-ref param as by-value (and e.g. emits memcpy(box, &ptrvar, ...), copying the pointer's address). Classify from the Type / type_struct_pass_by_ptr, never from the ctype string.

Where to look next

• docs/archive/end-to-end-monomorphization-plan.md -- the original rationale and "why-monomorphization" framing.
• docs/archive/end-to-end-monomorphization-plan-2.md -- the remaining-work plan, archived complete 2026-06-19. Has the detailed per-phase progress.
• docs/archive/m4-typeclass-per-method-abi-plan.md and docs/archive/m5-scope-audit-2026-06-18.md -- the per-method ABI and constrained-poly HOF sub-plans.
• docs/archive/m7-stdlib-migration-execution.md -- the by-value HKT stdlib migration log.
• docs/guides/hkt-guide.md -- the user-facing HKT story; this guide is the ABI underneath it.
• docs/guides/name-mangling-guide.md -- the full mangler rules, of which the spec-suffix rules in this guide are one piece.
• docs/guides/c-integration-guide.md -- inline-C, #[used], FFI patterns; this guide is the type story behind it.