Language specification

This document specifies the Yona language as implemented by the reference compiler yonac. Normative rules are stated in prose; implementation notes describe how yonac realizes them and are informative, not binding on other implementations.

A Yona program is a single expression. Compiling a source file that contains an expression produces an executable whose exit code is the expression’s value (for integer results). A source file may instead contain a module declaration, which compiles to a linkable object file plus an interface file (§7).

1. Lexical structure

1.1 Source text

Source files are UTF-8. Identifiers are ASCII: functions and variables match [a-z_][A-Za-z0-9_]* (camelCase by convention), type and constructor names and module segments match [A-Z][A-Za-z0-9_]* (PascalCase).

1.2 Comments

# a line comment runs to end of line
## a doc comment — extracted by the API documentation generator

/* a block comment
   /* block comments nest */
   and may span lines */

# introduces a line comment. ## at the start of a line is a documentation comment, attached to the following declaration by documentation tooling; to the compiler it is an ordinary comment. /* … */ comments nest and may contain newlines.

1.3 Newlines

Newlines are significant tokens. A newline (or a ;, which is equivalent) terminates an expression in the three positions where expression sequences occur: case arms, do-block steps, and module-level function bodies.

A newline is suppressed — treated as ordinary whitespace — in exactly these situations:

1. Inside brackets (), [], {}. Bracketed expressions may span any number of lines. Exception: when a case, do, with, or handle block is open inside the brackets, newlines again act as clause separators, so the block’s arms still terminate correctly.
2. After a binary operator or continuation token (+, *, ->, =, ,, |>, …). This permits natural line continuation:

let total = price +
            tax
in total

This rule is what allows juxtaposition application (§3.6) to coexist with expression sequences: f x y never runs onto the next line accidentally, because the newline ends it unless an operator invites continuation.

1.4 Keywords

let in do end case of if then else with as
module import export from type trait instance
try catch raise extern async daemon
perform handle resume effect for

1.5 Literals

Form	Examples	Notes
Integer	42, -17, 1_000_000	64-bit signed; _ separators permitted between digits
Float	3.14, -0.5, 1.23e-4	IEEE 754 double
String	"hello", "a\nb"	UTF-8; escapes \" \\ \n \r \t \0; interpolation §3.2
Character	'a', '\n'	single Unicode scalar
Boolean	true, false
Unit	()	the empty tuple; the type and value of “nothing”
Symbol	:ok, :not_found	interned atoms, snake_case by convention

Implementation note. Symbols are interned to 64-bit integer identifiers at compile time; symbol comparison is a single integer comparison, and matching on symbols compiles to an integer switch.

2. Values and their syntax

2.1 Collections

[1, 2, 3]                      # sequence (persistent list)
[]                             # empty sequence
(1, "two", true)               # tuple — fixed arity, heterogeneous
(42,)                          # one-element tuple
{1, 2, 3}                      # set
{"name": "Ada", "age": 36}     # dictionary
{}                             # empty dictionary

Sequences, sets, and dictionaries are persistent: every operation returns a new value sharing structure with the old one. Tuples are fixed-arity product values.

Implementation note. Small sequences are flat arrays; large ones are radix-balanced tries. Dictionaries and sets are hash array mapped tries (HAMTs). All share structure on update. See Persistent data structures.

2.2 Generators (comprehensions)

[x * 2 for x = xs]                     # sequence generator
[x for x = xs, if x > 3]               # with guard
{x * 2 for x = xs}                     # set generator
{k : v * 10 for k = ks}                # dictionary generator
[| f x for x = xs ]                    # parallel generator (§6.3)

The general form is [expr for pattern = source] with an optional , if guard. The source is any sequence-valued expression.

Implementation note. Generators compile to counted loops, not closures. With a guard, a two-pass strategy first counts matches, then fills the result without reallocation. Chained collection pipelines are stream-fused into a single loop when the compiler can prove it safe.

3. Expressions

3.1 let

let x = 42 in x + 1                       # single binding
let x = 10, y = 20 in x + y               # multiple bindings
let add x y = x + y in add 3 4            # function-definition binding
let (a, b) = (1, 2) in a + b              # pattern binding
let _ = println "side effect" in 42       # discard binding

let bindings in body introduces bindings scoped to body. Bindings are separated by commas. A binding’s left side is a pattern; a name followed by parameter patterns is sugar for binding a lambda. A type annotation may precede a function binding on its own line:

let add : Int -> Int -> Int
    add x y = x + y
in add 3 4

Evaluation order. Bindings that depend on earlier bindings observe their values. Bindings that are independent of one another have no defined sequential order and may be evaluated concurrently (§6.2). Code whose side effects require an order must use do.

3.2 Strings and interpolation

Within a string literal, {name} interpolates a variable and {(expr)} interpolates a parenthesized expression; non-string values are converted to their textual form:

let x = 6 in "the answer is {(x * 7)}"    # "the answer is 42"

3.3 do

do
    fd = tcpConnect "localhost" 8080     # binding step
    send fd "hello"                      # expression step
    response = recv fd 4096
    response                             # value of the block
end

do … end evaluates its steps strictly in textual order. A step of the form name = expr binds name for subsequent steps. The block’s value is its last expression. do is the sequencing primitive; use it whenever side effects must happen in order.

3.4 if

if x > 0 then "positive"
else if x < 0 then "negative"
else "zero"

if is an expression; both branches are required and must have the same type.

3.5 Functions and lambdas

add x y = x + y                # space-separated parameter patterns
factorial n = case n of
    0 -> 1
    _ -> n * factorial (n - 1)
end

abs x if x >= 0 = x            # guard; next clause if false
abs x if x < 0  = -x

scale : Float -> Float -> Float    # optional annotation
scale factor x = factor * x

\x -> x * 2                    # lambda
\(x, y) -> x + y               # lambda with tuple pattern
\-> expensive ()               # zero-argument lambda (thunk)

There is no name(x, y) -> body definition form. name (x, y) = body is a single tuple pattern, not two parameters.

A function of several clauses is matched top to bottom; the first clause whose patterns (and guard, if present) match is selected. Functions are first-class values; partial application is automatic:

let add5 = add 5 in add5 10    # => 15

Zero-arity functions auto-evaluate when referenced by name (Yona is strict). To pass one as a value, wrap it in a thunk: \-> f.

3.6 Application

Application is by juxtaposition — f x y. f(x) is the same as f x. f(x, y) applies f to the tuple (x, y); it is not a two-argument call. Juxtaposition binds tighter than every binary operator: f x + g y parses as (f x) + (g y).

Pipes reverse application order for pipeline style:

value |> stage1 |> stage2      # stage2 (stage1 value)
stage2 <| stage1 <| value      # the same, right-to-left

Implementation note. yonac compiles functions by deferred monomorphization: a definition is stored as a typed AST and compiled at each call site where concrete argument types are known. Closures capture free variables in a heap environment; a closure value is {fn_ptr, ret_tag, arity, captures…}.

3.7 case

case value of
    0 -> "zero"
    n if n > 0 -> "positive"
    _ -> "negative"
end

case scrutinee of clauses end evaluates the scrutinee once, then tests clauses top to bottom (§4 defines patterns). The first matching clause’s body is the expression’s value. All clause bodies must have the same type. If no clause matches at runtime, the program aborts with a match error; the compiler warns when it can prove a constructor uncovered.

3.8 with (resources)

with handle = tcpConnect "localhost" 8080 in
    send handle "hello"
# handle is closed when the body completes

with name = resource in body evaluates resource, binds it to name, evaluates body, and then releases the resource by calling the Closeable trait’s close method — resolved statically for the resource’s type. Using a value whose type does not implement Closeable is a compile-time error.

Current limitation. Release is guaranteed when body completes normally. If an exception propagates out of body, close is not currently invoked on the unwind path.

3.9 Exceptions

type Error = NotFound String | IOError String

raise (NotFound "config.toml")

try
    riskyOperation ()
catch
    NotFound path -> "missing: " ++ path
    IOError msg   -> "io: " ++ msg
    _             -> "unknown failure"
end

Exception values are ordinary ADT values. raise throws; try … catch … end matches the raised value against clauses like a case. An unmatched exception propagates; an uncaught exception terminates the program with a stack trace.

3.10 Algebraic effects Partial

handle
    perform State.get ()
with
    State.get () resume -> resume 42
    return val -> val
end

perform Effect.op arg requests the operation Effect.op from the nearest enclosing handle that covers it. A handler clause receives the operation’s argument and a resume continuation; return val -> … transforms the handled expression’s normal result.

Function types carry a latent effect row listing the operations the function may perform: Int -> !{State.get} Int. handle subtracts the operations it covers; applying a function whose row is not fully covered at the top level is error E0202, reported at the introducing perform with a note at the call site. Higher-order functions carry open rows (!{|r}) that unify with their argument’s row.

Current limitations. Handlers are shallow, in-scope dispatch: resume is an identity continuation, not a captured delimited continuation. effect declarations do not parse yet; operations are identified by their Effect.op label at perform sites. See Effects for the practical guide.

3.11 extern (C FFI)

extern sqrt : Float -> Float in
extern pow : Float -> Float -> Float in
sqrt (pow 2.0 10.0)            # => 32.0

extern async readFile : String -> String in
readFile "data.txt"            # non-blocking; auto-awaited at use

extern name : Type in body declares an external C symbol with a Yona type; the linker resolves it. Type mapping: Int ↔ i64, Float ↔ double, Bool ↔ i1, String ↔ char*. Curried annotation A -> B -> C denotes a two-argument C function returning C. The async modifier submits the call to the runtime’s thread pool and yields a promise, awaited transparently at use sites (§6.2).

4. Patterns

Pattern	Example	Matches
Literal	42, "hi", :ok, true	that exact value
Variable	x	anything; binds x
Wildcard	_	anything; binds nothing
Tuple	(a, _, c)	tuples of that arity
Sequence	[], [x], [a, b]	sequences of that exact length
Head–tail	[h | t], [a, b | rest]	non-empty sequences; t/rest bind the remainder
Constructor	Some x, Rect w h	values built by that constructor
Record	Person{name: n}	matches named fields; others ignored
Dictionary	{"key": v}	dictionaries containing the key
As-binding	[h | t] as whole	matches the inner pattern and binds the whole value
Or-pattern	:a | :b -> …	either alternative; both must bind the same names
Guard	n if n > 0 -> …	pattern matches and guard is true

Patterns appear in case clauses, function parameters, let bindings, and catch clauses. Matching is left-to-right, top-to-bottom, with no backtracking within a clause.

5. Operators

Precedence, highest to lowest; all binary operators are left-associative except **, ::, and the arrows:

Level	Operators	Meaning
1	.	field access
2	juxtaposition	function application
3	**	power
4	! ~ unary -	logical not, bitwise not, negation
5	* / %	multiplicative
6	+ -	additive
7	<< >> >>>	shifts
8	++	concatenation (sequences, strings)
9	::	cons (prepend)
10	< > <= >=	comparison
11	== !=	equality
12	&	bitwise and
13	^	bitwise xor
14	|	bitwise or
15	&&	logical and (short-circuit)
16	||	logical or (short-circuit)
17	|> <|	pipes

Sequence-specific operators: x :: xs prepends and xs ++ ys concatenates. The lexer reserves :> (append), -- (remove), and in (membership) as operator tokens, but the current compiler does not accept them in expressions — use xs ++ [x], Std\List.filter, and Std\List.contains (or Std\Set.contains) instead.

6. Evaluation model

6.1 Strictness

Yona is strictly evaluated: arguments are evaluated before application, and bindings before their bodies — with the single systematic exception of asynchronous values (§6.2). There is no lazy evaluation; laziness is expressed explicitly with thunks (\-> e) or Iterator/Std\Stream pipelines.

6.2 Transparent asynchrony

Functions that perform I/O (and extern async functions) return a promise internally. The type system tracks promises invisibly: when a promise appears where its underlying value is required — as an operator operand, function argument, or condition — the compiler inserts an await coercion. Users never write async or await, and no function is “colored”.

Because let bindings without mutual dependencies have no defined order, independent asynchronous bindings are submitted before any is awaited:

let
    a = readFile "foo.txt",    # submitted
    b = readFile "bar.txt"     # submitted
in a ++ b                      # both awaited here; elapsed ≈ max, not sum

Implementation note. On Linux, file and network I/O submit to io_uring; CPU-bound async work runs on a work-stealing thread pool. Buffers passed to in-flight kernel operations are pinned. See Concurrency in depth.

6.3 Parallel generators

[| f x for x = xs ] evaluates f over the elements concurrently on the thread pool and preserves order in the result.

6.4 Memory

Values are managed by atomic reference counting with recursive destructors; there is no tracing garbage collector and no stop-the-world pause. The compiler applies Perceus-style ownership transfer (callee-owns calling convention), uniqueness-based in-place mutation for uniquely owned values, and escape analysis that arena-allocates values proven not to escape. See Memory and linearity.

7. Types

7.1 Inference

The type system is Hindley–Milner: every expression has a principal type, inferred without annotations. Optional annotations (name : Type preceding a definition) are checked, not trusted. Polymorphic functions are compiled by monomorphization — one native instantiation per concrete type used.

7.2 Algebraic data types

type Option a = Some a | None
type Result a e = Ok a | Err e
type Tree a = Leaf | Node (Tree a) a (Tree a)
type Lazy a = Cons a (() -> Lazy a) | Empty       # function-typed field
type Person = Person { name : String, age : Int } # named fields

Constructors are first-class functions. Named-field types support dot access, record patterns, and functional update:

let p = Person { name = "Ada", age = 36 } in
(p.name, p { age = 37 })

Implementation note. Non-recursive ADTs compile to flat {tag, payload} structs; recursive ADTs and ADTs with function-typed fields are heap-allocated.

7.3 Traits Stable

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

instance Show a => Show (Option a)
    show opt = case opt of
        Some x -> "Some(" ++ show x ++ ")"
        None   -> "None"
    end
end

trait Eq a => Ord a                               # superclass constraint
    compare : a -> a -> Int
end

Traits are type classes resolved statically: each call site compiles the concrete instance directly (monomorphization), with no runtime dispatch cost. Instances are always public; export trait Name exports a declaration. See Traits.

7.4 Effect rows Partial

Function arrows carry the set of effect operations the function may perform: a -> !{State.get} Int. Rows are inferred, unioned at application, subtracted by handle, propagated through .yonai interfaces, and kept open (!{|r}) on higher-order parameters. §3.10 lists current limitations.

7.5 Linear types Partial

Linear a marks values that must be consumed exactly once: file handles, sockets, process handles, channel endpoints. The linearity checker rejects duplication and silent dropping; with is the idiomatic consumer. @borrow marks parameters that use a linear value without consuming it. See Memory and linearity.

7.6 Row-polymorphic records Stable

Record types unify by row: a function using r.name accepts any record containing a name field of the right type, and the residual row is polymorphic.

8. Modules

module Data\Geometry

export area, perimeter
export type Shape

type Shape = Circle Float | Rect Float Float

area s = case s of
    Circle r -> 3.141592653589793 * r * r
    Rect w h -> w * h
end

perimeter s = case s of
    Circle r -> 2.0 * 3.141592653589793 * r
    Rect w h -> 2.0 * (w + h)
end

A module is a top-level declaration — not an expression — and extends to end of file. export statements name exported functions; export type T exports a type with all its constructors; export f from Other\Module re-exports. Module names are backslash-separated paths (Std\List).

Imports are expressions:

import map, filter from Std\List in …        # selective
import length as len from Std\String in …    # aliased
import Std\Math in …                         # whole module
Std\List::map (\x -> x + 1) [1, 2, 3]        # fully qualified, no import

Implementation note. A module compiles to a native object file with C-ABI exports (mangled yona_Pkg_Mod__func) and a .yonai interface file carrying types, effect rows, linearity, and — for generic functions — the source text itself (GENFN), so a caller with new concrete types can re-monomorphize the function locally. yonac -I path adds interface search paths. See Modules and interfaces.

9. Conformance and diagnostics

Compiler diagnostics carry stable codes (E0100-style, W… for warnings). yonac --explain E0202 prints the full explanation for a code. The error code index lists user-facing codes and their meanings.