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https://github.com/jasisz/aver

Aver is a programming language for auditable AI-written code: verify in source, deploy with Rust, prove with Lean/Dafny
https://github.com/jasisz/aver

ai-code dafny effect-system formal-verification lean4 programming-language rust spec-tests transpiler

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Aver is a programming language for auditable AI-written code: verify in source, deploy with Rust, prove with Lean/Dafny

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README 

          


  

    

    

    aver

  




# Aver



Aver is a statically typed language designed for AI to write in and humans to review, with a bytecode VM for runtime execution, a Rust backend for deployment, a WASM backend for browser and embedded targets, Lean proof export for pure logic and classified effectful laws, and Dafny verification for automated law checking via Z3.



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

- selected effectful behavior can be verified with explicit stubs and trace assertions

- effectful runs can be recorded and replayed deterministically

- `aver why` scores every function's justification coverage: description, verify, decisions

- `aver audit` runs all three axes — static checks, verify execution, format compliance — as a single CI gate

- `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 or a standalone WASM module

- `aver proof` exports the pure subset of an Aver module graph to a Lean 4 proof project (default) or Dafny verification file (`--backend dafny`)



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.



Website: [averlang.dev](https://averlang.dev)  

Browser playground: [averlang.dev/playground](https://averlang.dev/playground/)



Read the [Aver Manifesto](https://jasisz.github.io/aver-language/) for the longer argument, [Common Pushback](docs/pushback.md) for questions and objections, or [intent-trace](https://github.com/jasisz/intent-trace) for an empirical benchmark comparing Aver's legibility against Python variants across 108 AI-reviewed code changes.



---



## Quickstart



### Install from crates.io



```bash

cargo install aver-lang

```



Then try it with a tiny file:



```bash

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 run      hello.av --self-host

aver verify   hello.av

aver verify   hello.av --json

aver check    hello.av

aver check    hello.av --json

aver audit    hello.av

aver audit    hello.av --json

aver why      hello.av

aver context  hello.av

aver compile  hello.av -o out/

(cd out && cargo run)

```



`aver run`, `verify`, `check`, and `replay` use the bytecode VM by default. The old

`--vm` flag is gone.



`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`; `aver run` treats `Result.Err(...)` from `main` as a process failure.



### Build from source



```bash

git clone https://github.com/jasisz/aver

cd aver

cargo install --path . --force



aver run      examples/core/calculator.av

aver run      examples/core/calculator.av --self-host

aver verify   examples/core/calculator.av

aver check    examples/core/calculator.av

aver audit    examples/core/calculator.av

aver why      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 run      examples/core/calculator.av -e 'safeDivide(10, 2)' --record recordings/

aver replay   recordings/ --test --diff

aver replay   recordings/ --test --check-args

aver replay   recordings/ --test --json

```



Requires: Rust stable toolchain.



### Editor support



For editor integration:



```bash

cargo install aver-lsp

```



Then install the VS Code extension `Aver.aver-lang`, or configure your editor to start the `aver-lsp` binary directly. See [editors/README.md](editors/README.md) for VS Code, Sublime Text, and manual LSP setup notes.



---



## Small example



```aver

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

    ? "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

- no async/await, streams, or channels — `(a, b)?!` declares independent computations; the runtime handles the rest. See [docs/independence.md](docs/independence.md)



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](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 execution happens through the bytecode VM during development, with deployment through Rust code generation or WASM compilation.



---



## Execution modes



### Default (VM)



```bash

aver run hello.av

```



All commands use the bytecode VM. For VM internals, see [docs/vm.md](docs/vm.md).



### WASM



```bash

# Compile to .wasm (requires --features wasm)

aver compile hello.av --target wasm



# Compile with WASI adapter for standalone wasmtime

aver compile hello.av --target wasm --adapter wasi



# Run via built-in WASM host

aver run hello.av --wasm

```



See [docs/wasm.md](docs/wasm.md) for ABI, memory model, and browser hosting.



### Native Rust



```bash

aver compile hello.av              # generates Rust/Cargo project

aver compile hello.av --policy embed   # bake aver.toml into binary

aver compile hello.av --policy runtime # load aver.toml at runtime

```



See [docs/rust.md](docs/rust.md) for the generated code model.



### Self-hosted



```bash

aver run hello.av --self-host

```



Runs through the Aver interpreter written in Aver itself, compiled to Rust and cached on demand. See [self_hosted/README.md](self_hosted/README.md).



---



## What Aver makes explicit



### Effects



```aver

fn processPayment(amount: Int) -> Result

    ? "Validates and records the charge. Pure — no network, no disk."

    match amount

        0 -> Result.Err("Cannot charge zero")

        _ -> Result.Ok("txn-{amount}")

```



```aver

fn fetchExchangeRate(currency: String) -> Result

    ? "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 can be granular or namespace-wide:



- `! [Http.get]` allows only `Http.get`

- `! [Http]` covers all `Http.*` effects



`aver check` suggests narrowing a broad namespace effect when a function only needs a smaller subset.



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



```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?



Generated Rust can use the same scoped runtime machinery when you compile with `--with-replay`; see [docs/rust.md](docs/rust.md).



### Decisions



```aver

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:



```bash

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

```



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



### Context export



```bash

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` with priority scoring: elements with more verify coverage, spec references, and decisions are included first. `--depth N` and `--depth unlimited` bypass that budget. Long verify examples are skipped rather than bloating the artifact.



Use `--focus ` to build context around a specific function — its callees, types, verify blocks, and decisions are prioritized within the budget:



```bash

aver context examples/data/json.av --focus fromString

```



This makes token budget a navigation primitive. Another human or model can start with a small architecture map, zoom into the modules that matter, or focus on a single function's dependency cone.



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



```bash

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:



```text

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:



```markdown

## Module: Calculator

> Safe calculator demonstrating Result types, match expressions, and co-located verification.



### `safeDivide(a: Int, b: Int) -> Result`

> 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



```aver

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.



Regular verify:



```aver

verify add

    add(1, 2) => 3

    add(0, 0) => 0

```



Law verify:



```aver

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](docs/language.md) and [docs/lean.md](docs/lean.md).



Oracle laws for classified effects:



```aver

fn fairDie(path: BranchPath, n: Int, min: Int, max: Int) -> Int

    ? "Deterministic Random.int stub."

    4



fn pickOne() -> Int

    ? "Rolls once."

    ! [Random.int]

    Random.int(1, 6)



verify pickOne law usesOracle

    given rnd: Random.int = [fairDie]

    pickOne() => rnd(BranchPath.Root, 0, 1, 6)

```



`verify  law ` is the proof-oriented Oracle form: `aver proof` lifts the effectful function to a pure function with explicit oracle parameters and can emit a universal theorem over that oracle. Use cases-form `verify  trace` when you want runtime assertions over `.result` and `.trace.*`, such as `trace.contains(Random.int(1, 6))`. See [docs/oracle.md](docs/oracle.md) for the full model.



### Replay



Use deterministic replay for stateful, interactive, or external effectful code:



1. run once against real services and record the effect trace

2. replay offline with no real network, disk, or TCP calls

3. use `--diff` and `--test` to turn recordings into a regression suite



```bash

aver run    examples/services/console_demo.av --record recordings/

aver replay recordings/rec-123.json --diff

aver replay recordings/ --test --diff

```



Use Oracle laws when classified effects should become proof obligations over explicit stubs. Use `verify  trace` when you need runtime assertions over `.result` or `.trace.*`. Use replay when the flow depends on ambient state, persistent TCP sessions, terminal modes, or server callbacks.



---



## 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` with priority scoring. Use `--focus ` to zoom into a function's dependency cone, `--budget 24kb` for a larger export, or `--depth unlimited` for the full graph.



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](docs/language.md).



For constructor rules and edge cases, see [docs/constructors.md](docs/constructors.md).



For namespaces, effectful services, and the standard library, see [docs/services.md](docs/services.md).



## Execution and proof backends



Aver has four backend paths:



- VM-based 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, Oracle-lifted classified effectful laws, and `verify` / `verify law` obligations with `aver proof`

  Supported law shapes become real universal theorems; the rest stay as

  executable samples or checked-domain theorems instead of fake proofs.

- Dafny verification for automated `verify law` checking via Z3 with `aver proof --backend dafny`



The VM and generated Rust share practical behavior through `aver-rt`: 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:



```bash

aver compile examples/core/calculator.av -o out/

cd out

cargo run

```



Typical Lean flow:



```bash

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

cd out

lake build

```



Typical Dafny flow:



```bash

aver proof examples/data/fibonacci.av --backend dafny -o out/

cd out

dafny verify fibonacci.dfy

```



Rust is the deployment backend. Lean and Dafny are complementary proof backends:



- **Lean** handles `verify` cases via `native_decide` (100% success on concrete examples) and supported `verify law` shapes via hand-crafted tactic strategies

- **Dafny** emits `verify law` blocks as lemmas and lets Z3 attempt automated proofs — no tactic authoring needed, but limited on concrete computation



For backend-specific details, see:

- [docs/rust.md](docs/rust.md) for Cargo generation and deployment flow

- [docs/lean.md](docs/lean.md) for proof export, formal-verification path, and current Lean examples

- [docs/dafny.md](docs/dafny.md) for Dafny verification and Z3-powered law checking

- [docs/oracle.md](docs/oracle.md) for Oracle laws, classified-effect stubs, and trace assertions



---



## 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

- `games/` for interactive terminal games (Snake, Tetris, Braille Doom)

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:



```bash

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

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

aver check projects/payment_ops/main.av --module-root projects/payment_ops --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`, conservative universal auto-proofs plus sampled/domain fallback |

| `formal/spec_laws.av` | Implementation-vs-spec laws (`verify foo law fooSpec`) and Lean spec theorems for supported shapes |

| `formal/oracle_trace.av` | Oracle laws and trace assertions for classified effects with explicit stubs |

| `apps/mission_control.av` | Command parser, pure state machine, effectful shell |

| `games/wumpus.av` | Hunt the Wumpus: cave exploration, match-driven control flow |

| `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 |

| `games/snake.av` | Terminal Snake: immutable state, TCO game loop, Terminal service |

| `games/tetris/main.av` | Modular Tetris: sum types, 2D grid, collision, line clearing (4 modules, 66 verify cases) |

| `games/checkers/main.av` | Checkers with alpha-beta AI: cursor UI, forced capture, decision trace (5 modules, 144 verify cases) |

| `games/doom/main.av` | Braille Doom: raycasting FPS with Unicode Braille rendering, procedural levels, 3 enemy types with AI, shooting, wall textures (7 modules) |

| `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 |

| `projects/payment_ops/main.av` | Dirty payment backoffice flow: provider normalization, replay, settlement reconcile, manual-review cases, audit trail |

| `self_hosted/main.av` | Full self-hosted interpreter: lexer, parser, resolver, evaluator — all 55 examples pass. Compiles to native via `aver compile` and powers `aver run --self-host`. |



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](docs/language.md) | Surface-language guide: syntax, semantics, modules, and deliberate omissions |

| [docs/formatting.md](docs/formatting.md) | Formatter behavior and guarantees |

| [docs/constructors.md](docs/constructors.md) | Constructor rules and parsing contract |

| [editors/README.md](editors/README.md) | VS Code + LSP setup and Sublime Text support |

| [docs/services.md](docs/services.md) | Full API reference for all namespaces (signatures, effects, notes) |

| [docs/oracle.md](docs/oracle.md) | Oracle: effectful laws, trace assertions, and proof export for classified effects |

| [docs/vm.md](docs/vm.md) | Bytecode VM design note: execution model, `NanValue`, opcodes, effects |

| [docs/types.md](docs/types.md) | Key data types (compiler, AST, runtime) |

| [docs/extending.md](docs/extending.md) | How to add keywords, namespace functions, expression types |

| [docs/transpilation.md](docs/transpilation.md) | Overview of `aver compile` and `aver proof` |

| [docs/rust.md](docs/rust.md) | Rust backend: deployment-oriented Cargo generation |

| [docs/wasm.md](docs/wasm.md) | WASM backend: ABI, memory model, browser hosting |

| [docs/lean.md](docs/lean.md) | Lean backend: proof export and formal-verification path |

| [docs/dafny.md](docs/dafny.md) | Dafny backend: Z3-powered automated law verification |

| [docs/independence.md](docs/independence.md) | Independent products: the semantic model behind `?!` and `!` |

| [docs/research.md](docs/research.md) | Narrow related work for effects, Oracle, independent products, and proof targets |

| [docs/pushback.md](docs/pushback.md) | Common pushback: questions, objections, and honest answers |

| [docs/decisions.md](docs/decisions.md) | Decision export generated via `aver context --decisions-only` |