The type system

Yona is statically typed with full type inference. You rarely write a type annotation; the compiler reconstructs principal types, monomorphizes generic code to concrete machine code, and layers several extensions on the core: algebraic data types, traits, row-polymorphic records, effect rows, linear types, and refinement checks. This page maps the whole system and is honest about which layers are complete and which are still partial.

Status badges next to each heading reflect the current implementation, not the design goal.

Hindley–Milner core Stable

The foundation is Hindley–Milner inference with let-polymorphism. Every well-typed expression has a principal type — the most general type of which every other valid type is an instance — and the compiler finds it without annotations:

let twice f x = f (f x) in
twice (\n -> n + 1) 40              # => 42

twice is inferred as (a -> a) -> a -> a. A let-bound function is generalized over the type variables not fixed by its environment and may be used at several different types in the same scope:

let pair x y = (x, y) in
(pair 1 2, pair "a" "b")            # => ((1, 2), ("a", "b"))

Monomorphization

Polymorphism is a compile-time phenomenon only. At each call site the compiler instantiates the function at the concrete argument types and compiles a specialized native version — the same strategy as Rust generics or C++ templates, and unlike the uniform boxed representation of OCaml or Haskell. Polymorphic code therefore pays no boxing, no tags, and no dynamic dispatch at runtime.

Implementation note. Functions are stored as AST at definition and compiled at the call site where argument types are known. Exported generic functions carry their source text in .yonai interface files so importing modules can re-instantiate them at new types (see Modules and interfaces).

Algebraic data types Stable

ADTs declare a closed set of constructors with typed fields. Constructors are first-class functions, and pattern matching is the elimination form:

type Shape = Circle Float | Rect Float Float

let area s = case s of
    Circle r   -> 3.14159 * r * r
    Rect w h   -> w * h
end in
area (Rect 3.0 4.0)                  # => 12.0

Fields may have function types (type Lazy a = Cons a (() -> Lazy a) | Empty), and types may be recursive and polymorphic. The prelude types Option a, Result a e, Linear a, and Iterator a are ordinary ADTs.

Exhaustiveness Partial

The compiler warns when an Option/Result/other ADT value is silently discarded in a do block or bound to _ (-Wunmatched-adt, enabled by --Wall). It also warns for a finite-ADT case that misses constructors (--Wincomplete-patterns, also enabled by --Wall). A wildcard arm closes coverage; a guarded arm does not. --require-effect-free turns missing alternatives in those finite-ADT and Bool cases into E0203 errors, alongside its closed-empty effect-row requirement and conservative direct structural recursion check. Mutual recursion is rejected. --Woverlapping-patterns warns about a later arm provably covered by earlier unguarded arms; it follows nested constructors, tuples, sequence shapes, aliases, alternatives, and literals. The strict gate does not prove general termination or arbitrary non-ADT coverage.

Traits Stable

Traits are type classes resolved entirely at compile time — static dispatch, superclass constraints (trait Eq a => Ord a), default methods, constrained instances (instance Show a => Show (Option a)), and multi-parameter traits:

trait Eq a
    eq  : a -> a -> Bool
    neq : a -> a -> Bool
    neq x y = if eq x y then false else true    # default method
end

Because dispatch is monomorphized, trait calls cost the same as direct calls — and there are no runtime trait objects. The full treatment, including auto-derive, is in Traits.

Record-row polymorphism Stable

Record types unify by row: a function that reads a field accepts any record that has that field, and the rest of the record stays polymorphic:

let greet r = "hello, " ++ r.name in
greet { name = "Alice", age = 30 }   # => "hello, Alice"

greet is inferred as { name : String | r } -> String — the row variable r stands for “whatever other fields the record has”. Missing fields and field type mismatches are compile-time errors. Record rows are structural; they are distinct from the effect rows on function arrows below.

One limitation: open row variables on records are not yet printed into .yonai interface signatures, so cross-module functions may show concretized record types even where the checker inferred an open row.

Effect rows Partial

Function arrows carry a latent effect row — the set of effect operations the function may perform, written !{Effect.op}:

(\x -> perform State.get ())         # : a -> !{State.get} Int

The rules:

• perform Effect.op adds the label to the ambient row when no enclosing handle covers it.
• handle … with … subtracts the operations its clauses cover; anything left escapes to the outer row.
• Application unions the callee’s latent row into the caller’s row. At the top level of a program, applying a function whose row is not fully handled is error E0202, reported at the introducing perform with a note at the call site.
• Higher-order functions preserve every callback source separately. For use f g n = (f n, g n), the inferred row is the union of f and g’s rows; using the helper again instantiates fresh effect variables rather than equating the callbacks.
• Handlers subtract operations symbolically, so a handler can mask an effect that is still open when a helper is defined and only becomes concrete at its call site.
• Recursive definitions use least-derived effect cells. Pure direct and mutual recursive components close; callback and imported opaque sources remain open rather than being defaulted away.

# The call site must handle the latent effects of f
handle f 0 with
    State.get () resume -> resume 7
    return val -> val
end                                   # => 7

Rows survive module boundaries. .yonai files include the canonical effectscheme normal form for every arrow, shared open source, and handler mask; import clones that graph so polymorphic siblings stay independent. The readable effects Fs.read field is the normalized summary. A missing effects field means unknown, not pure.

Honest limitations: effect Name … end declarations do not parse yet (an operation’s identity is its Effect.op label at the perform site), handlers are shallow in-scope dispatch rather than captured delimited continuations, and an empty row is not yet usable as a totality/purity guarantee. See Effects for the practical guide.

Linear types Partial

Linear a marks a value that must be consumed exactly once — file handles, sockets, process handles, channel endpoints. Pattern matching on the Linear constructor is the consumption point; rebinding transfers the obligation:

let conn = Linear (tcpConnect "host" 8080) in
let conn2 = conn in                  # obligation transferred to conn2
send conn "hello"                    # error E0600: conn already consumed

A flow-sensitive linearity checker tracks each linear binding as live or consumed, requires branches of if/case to agree on what they consume (E0601), and warns when a linear value is still live at scope exit — a resource leak. with is the idiomatic consumer and discharges the obligation automatically. Details and examples are in Memory and linearity.

Honest limitations: Linear is a prelude ADT tracked by a dedicated checker, not a first-class linear arrow in the HM core. Use-after-consume (E0600) and branch inconsistency (E0601) fail compilation on expression programs and modules (--Wno-linear skips). Resource leaks are E0602 warnings (-Wlinear-leak, on by default).

@borrow parameters Stable

@borrow before a parameter declares a read-only, non-escaping contract: the callee may use the value but not return it, store it, or capture it in a closure. The compiler verifies the contract (error E0603 on violation) and skips reference-count traffic for the parameter. Borrow information is inferred automatically even without the annotation; writing @borrow documents the contract and turns a future violation into a compile error instead of a silent deoptimization.

import foldl from Std\List in
let sum @borrow xs = foldl (\a b -> a + b) 0 xs in
sum [1, 2, 3]                        # => 6

Refinement types Partial

A refinement checker proves simple value-level facts and reports error E0500 when an operation’s precondition cannot be established: head/tail on a sequence not proven non-empty, and division by a value not proven non-zero.

let first xs = case xs of
    [h|t] -> h                       # h proven present by the pattern
    []    -> 0
end in
first [7, 8]                         # => 7

Facts flow from pattern matches ([h|t] proves non-empty) and literals (a non-zero literal divisor is accepted). Honest limitations: refinement syntax like { x : Int | x > 0 } parses but predicates are not enforced at function signatures, refinements are erased before codegen, and they do not appear in .yonai. yonac exits non-zero on E0500 (--Wno-refinement skips the checker) for both expression programs and modules.

What is checked where

Yona has two compilation entry points, and they differ in which passes run:

• Expression programs (a .yona file whose top level is an expression, or yona -e / yonac -): parse → HM type checking → refinement and linearity checkers (fail the compile on E0500/E0600/E0601) → codegen. All diagnostics described on this page can appear.
• Module compilation (a .yona file declaring module …): parse → HM type checking → the same refinement and linearity checkers → codegen, producing a native object file plus a .yonai interface.

The .yonai interface is the contract at module boundaries. It carries each export’s arity and types, effect rows (including open rests), inferred borrow masks, LINEAR markers on resource-producing functions, trait and instance tables, ADT definitions, and the source text of generic functions for cross-module monomorphization. Importers re-check calls against these signatures, so a type error at a module boundary is caught at the caller even though the callee was compiled separately.

Summary

Layer	Status
HM inference, principal types, monomorphization	Stable
ADTs and pattern matching	Stable
Case exhaustiveness diagnostics	Partial
Traits (static dispatch, superclasses, defaults)	Stable
Record-row polymorphism	Stable
Effect rows, E0202	Partial
Linear types, E0600/E0601	Partial
@borrow, E0603	Stable
Refinements, E0500	Partial

The language specification is the normative reference for the stable layers.