Aver

Aver is a statically typed language designed for AI to write in and humans to review, with a fast interpreter for iteration, a Rust backend for deployment, and Lean proof export for pure core logic.

It is built around one idea: the risky part of AI-written code is usually not syntax, it is missing intent. Aver makes that intent explicit and machine-readable:

effects are part of the function signature
decisions live next to the code they explain
pure behavior lives in colocated verify blocks
effectful behavior can be recorded and replayed deterministically
aver context exports the contract-level view of a module graph for humans or LLMs
aver compile turns an Aver module graph into a Rust/Cargo project
aver proof exports the pure subset of an Aver module graph to a Lean 4 proof project

This is not a language optimized for humans to type by hand all day. It is optimized for AI to generate code that humans can inspect, constrain, test, and ship.

Read the Aver Manifesto for the longer argument.

Quickstart

Install from crates.io

cargo install aver-lang

Then try it with a tiny file:

cat > hello.av <<'EOF'
module Hello
    intent =
        "Tiny intro module."
    exposes [greet]

fn greet(name: String) -> String
    ? "Greets a user."
    "Hello, {name}"

fn runCli() -> Unit
    ? "Starts the CLI."
      "Prints one rendered response."
    ! [Args.get, Console.print]
    Console.print("todo")

verify greet
    greet("Aver") => "Hello, Aver"

fn main() -> Unit
    ! [Console.print]
    Console.print(greet("Aver"))
EOF

aver run      hello.av
aver verify   hello.av
aver check    hello.av
aver context  hello.av
aver compile  hello.av -o out/
(cd out && cargo run)

Unit is Aver's "no meaningful value" type, roughly like void and rendered as () in diagnostics. main often returns Unit, but it can also return Result<Unit, String>; aver run treats Result.Err(...) from main as a process failure.

Build from source

git clone https://github.com/jasisz/aver
cd aver
cargo install --path . --force

aver run      examples/core/calculator.av
aver verify   examples/core/calculator.av
aver check    examples/core/calculator.av
aver context  examples/core/calculator.av
aver compile  examples/core/calculator.av -o out/
(cd out && cargo run)
aver proof    examples/formal/law_auto.av -o proof/
(cd proof && lake build)
aver run      examples/services/console_demo.av --record recordings/
aver replay   recordings/ --test --diff

Requires: Rust stable toolchain.

Small example

module Payments
    intent =
        "Processes transactions with an explicit audit trail."
    exposes [charge]

decision UseResultNotExceptions
    date = "2024-01-15"
    reason =
        "Invisible exceptions lose money at runtime."
        "Callers must handle failure — Result forces that at the call site."
    chosen = "Result"
    rejected = ["Exceptions", "Nullable"]
    impacts = [charge]

fn charge(account: String, amount: Int) -> Result<String, String>
    ? "Charges account. Returns txn ID or a human-readable error."
    match amount
        0 -> Result.Err("Cannot charge zero")
        _ -> Result.Ok("txn-{account}-{amount}")

verify charge
    charge("alice", 100) => Result.Ok("txn-alice-100")
    charge("bob",   0)   => Result.Err("Cannot charge zero")

No if/else. No loops. No exceptions. No nulls. No implicit side effects.

Deliberate constraints

Aver is intentionally opinionated. These omissions are part of the design, not missing features:

no if/else - branching goes through match
no for/while - iteration is recursion or explicit list operations
no exceptions - failure is Result
no null - absence is Option
no closures - functions are top-level and explicit

The point is to remove classes of implicit behavior that are easy for AI to generate and annoying for humans to audit.

For the fuller language rationale, see docs/language.md.

Why Aver exists

LLMs can produce function bodies quickly. They are much worse at preserving the information reviewers actually need:

what a function is allowed to do
why a design was chosen
what behavior must keep holding after a refactor
what a new human or model needs to understand the codebase without reading everything

Traditional languages usually push that into comments, external docs, stale tests, or team memory. Aver makes those concerns part of the language and tooling.

The intended workflow is explicit: AI writes Aver, humans review contracts and intent, and deployment happens either through the interpreter or by transpiling to Rust.

What Aver makes explicit

Effects

fn processPayment(amount: Int) -> Result<String, String>
    ? "Validates and records the charge. Pure — no network, no disk."
    match amount
        0 -> Result.Err("Cannot charge zero")
        _ -> Result.Ok("txn-{amount}")

fn fetchExchangeRate(currency: String) -> Result<HttpResponse, String>
    ? "Fetches live rate from the ECB feed."
    ! [Http.get]
    Http.get("https://api.ecb.europa.eu/rates/{currency}")

Effects such as Http.get, Disk.readText, and Console.print are part of the signature. Missing declarations are type errors. The runtime enforces the same boundary as a backstop.

Effects are exact:

! [Http.get] allows only Http.get
! [Http] does not cover Http.get

Runtime policy can narrow the allowed destinations further via aver.toml:

[effects.Http]
hosts = ["api.example.com", "*.internal.corp"]

[effects.Disk]
paths = ["./data/**"]

[effects.Env]
keys = ["APP_*", "PUBLIC_*"]

Think of this as two separate controls:

code answers: what kind of I/O is allowed?
policy answers: which concrete destinations are allowed?

Decisions

decision UseResultNotExceptions
    date = "2024-01-15"
    reason =
        "Invisible exceptions lose money at runtime."
        "Callers must handle failure — Result forces that at the call site."
    chosen = "Result"
    rejected = ["Exceptions", "Nullable"]
    impacts = [charge, refund, settle]
    author = "team"

decision blocks are first-class syntax, colocated with the code they explain.

Query only the decision history for a module graph:

aver context decisions/architecture.av --decisions-only

impacts, chosen, and rejected accept either validated symbols or quoted semantic labels.

Context export

aver context examples/core/calculator.av

Aver walks the dependency graph and emits a compact context summary: module intent, public signatures, effect declarations, verify samples, and decisions. The goal is not to dump the whole source tree; it is to export the contract-level view that another human or LLM needs first.

By default, aver context uses --depth auto --budget 10kb: it tries depth 0, 1, 2, ... and keeps the deepest result that still fits the byte budget. --depth N and --depth unlimited bypass that budget. Long verify examples are skipped rather than bloating the artifact.

This makes token budget a navigation primitive. Another human or model can start with a small architecture map, then zoom into the modules that matter instead of reading the whole tree up front.

If you want a larger export for a medium project, raise the budget explicitly:

aver context projects/workflow_engine/main.av \
  --module-root projects/workflow_engine \
  --json \
  --budget 24kb \
  --output projects/workflow_engine/CONTEXT.json

When --output is used, Aver also prints a short selection summary to stdout, for example:

mode auto, included depth 2, used 22622b, budget 24kb, truncated, next depth 3 would use 40739b

The same selection metadata is embedded in JSON output so you can see whether the export stopped because of the budget.

Example shape:

## Module: Calculator
> Safe calculator demonstrating Result types, match expressions, and co-located verification.

### `safeDivide(a: Int, b: Int) -> Result<Int, String>`
> Safe integer division. Returns Err when divisor is zero.
verify: `safeDivide(10, 2)` → `Result.Ok(5)`

## Decisions
### NoExceptions (2024-01-15)
**Chosen:** Result — **Rejected:** Exceptions, Nullable

Verify

verify charge
    charge("alice", 100) => Result.Ok("txn-alice-100")
    charge("bob",   0)   => Result.Err("Cannot charge zero")
    charge("x",    -1)   => Result.Ok("txn-x--1")

verify blocks stay next to the function they cover. aver check treats a missing verify block on a pure, non-trivial, non-main function as a contract error. Effectful flows are intentionally handled separately via replay.

Regular verify:

verify add
    add(1, 2) => 3
    add(0, 0) => 0

Law verify:

verify add law commutative
    given a: Int = -2..2
    given b: Int = [-1, 0, 1]
    add(a, b) => add(b, a)

verify ... law ... is deterministic, not random sampling. Cases are generated as the cartesian product of explicit domains, capped at 10_000.

For the proof-oriented style where a law relates an implementation to a pure spec function, see docs/language.md and docs/lean.md.

Replay

Use deterministic replay for effectful code:

run once against real services and record the effect trace
replay offline with no real network, disk, or TCP calls
use --diff and --test to turn recordings into a regression suite

aver run    examples/services/console_demo.av --record recordings/
aver replay recordings/rec-123.json --diff
aver replay recordings/ --test --diff

Pure logic belongs in verify. Effectful flows belong in replay recordings.

Common commands

aver check   file.av
aver run     file.av
aver verify  file.av
aver context file.av
aver compile file.av -o out/

aver verify runs only the example cases from verify blocks and fails on mismatches or example errors. aver check handles static contract diagnostics such as missing verify blocks and coverage-style warnings. Both check and verify accept --deps to walk transitive depends [...] modules.

For recursive code, aver check also warns when a recursive function still contains non-tail recursive calls after TCO. Tail-recursive code remains the preferred shape for large linear traversals; the warning is there to point out where accumulator-style rewrites may matter.

aver context defaults to --depth auto --budget 10kb. Use --budget 24kb, --depth 2, or --depth unlimited when you want a deeper export.

For replay, formatting, REPL, and the full command surface, use aver --help and the docs below.

Language and runtime

Aver is intentionally small. The core model is:

immutable bindings only
match instead of if/else
Result and Option instead of exceptions and null
top-level functions only, with no closures
explicit method-level effects
module-based structure via module, depends, and exposes
automatic memoization and tail-call optimization for eligible code

For the surface-language guide, see docs/language.md.

For constructor rules and edge cases, see docs/constructors.md.

For namespaces, effectful services, and the standard library, see docs/services.md.

Execution and proof backends

Aver has three backend paths:

interpreter-first workflow for run, check, verify, replay, and context
Rust compilation for generating a native Cargo project with aver compile
Lean proof export for pure core logic and verify / verify law obligations with aver proof

The interpreter and generated Rust now share more practical behavior through aver-rt than the name alone suggests: list teardown, deep append -> match paths, and string helpers such as String.slice are intentionally centralized there so one runtime fix can improve both execution paths.

Typical Rust flow:

aver compile examples/core/calculator.av -o out/
cd out
cargo run

Typical Lean flow:

aver proof examples/formal/law_auto.av --verify-mode auto -o out/
cd out
lake build

Rust is the deployment backend. Lean is the proof-export backend for the pure subset of Aver.

For backend-specific details, see:

docs/rust.md for Cargo generation and deployment flow
docs/lean.md for proof export, formal-verification path, and current Lean examples

Examples

Shared examples under examples/ resolve from --module-root examples. They are grouped by role:

core/ for language and syntax tours
data/ for pure data structures and parsers
formal/ for Lean-oriented proof examples
modules/ for import and module-root examples
services/ for effectful adapter demos
apps/ for small multi-file applications under the shared examples root Standalone multi-file showcase projects live under projects/ and use their own local module roots.

Repository layout rule:

files under examples/ share one root: --module-root examples
each folder under projects/ is its own root, for example --module-root projects/workflow_engine

Typical commands:

aver check examples/modules/app.av --module-root examples --deps
aver check projects/workflow_engine/main.av --module-root projects/workflow_engine --deps

Curated shared examples:

File	Demonstrates
`core/hello.av`	Functions, string interpolation, verify
`core/calculator.av`	Result types, match, decision blocks
`core/shapes.av`	Sum types, qualified constructors (`Shape.Circle`), match on variants
`data/fibonacci.av`	Tail recursion, records, decision blocks
`formal/law_auto.av`	Lean proof export, `verify law`, auto-proved universal theorems
`formal/spec_laws.av`	Implementation-vs-spec laws (`verify foo law fooSpec`) and Lean spec theorems
`apps/mission_control.av`	Command parser, pure state machine, effectful shell
`modules/app.av`	Module imports via `depends [Data.Fibonacci]`
`services/console_demo.av`	Console service and replay-friendly effectful flow
`services/http_demo.av`	HTTP service with sub-effects: `Http.get`, `Http.post`
`services/weather.av`	End-to-end service: `HttpServer` + `Http` + `Tcp`
`apps/notepad/app.av`	Multi-file HTTP app under the shared `examples` module root
`core/test_errors.av`	Intentional `aver check` failures: type errors + verify/decision/effect diagnostics

Standalone projects:

File	Demonstrates
`projects/workflow_engine/main.av`	Explicit app/domain/infra flow, event replay, derived events, verify-driven orchestration

See examples/ and projects/ for the full set. For repository self-documentation via decision exports, see decisions/architecture.av.

Documentation

Document	Contents
docs/language.md	Surface-language guide: syntax, semantics, modules, and deliberate omissions
docs/formatting.md	Formatter behavior and guarantees
docs/constructors.md	Constructor rules and parsing contract
editors/README.md	VS Code + LSP setup and Sublime Text support
docs/services.md	Full API reference for all namespaces (signatures, effects, notes)
docs/types.md	Key data types (compiler, AST, runtime)
docs/extending.md	How to add keywords, namespace functions, expression types
docs/transpilation.md	Overview of `aver compile` and `aver proof`
docs/rust.md	Rust backend: deployment-oriented Cargo generation
docs/lean.md	Lean backend: proof export and formal-verification path
docs/decisions.md	Decision export generated via `aver context --decisions-only`

aver-lang 0.5.0