Va is an emerging smart contract language for the Vapory blockchain.

Getting started

• Build the compiler

Overview

Fe is inspired by Vyper and aims to achieve the goals of the existing Vyper project along with the following additional goals:

• Emphasis on correct implementation via use of better tooling, application of formal methods, and stricter development practices
• A complete language specification to aid in verification of compiler correctness
• Support for both eWASM and EVM code generation via use of the YUL IR format
• Implementation in a powerful, systems-oriented language (Rust) with strong safety guarantees
• Syntactic improvements and additional code re-use features
• Statically built compiler binaries for smoother installation process
• WASM compiler binaries for enhanced portability and in-browser compilation of Fe contracts

Inherited goals

Fe aims to achieve most of the goals of the existing Vyper project. Those goals are described in that project's documentation though we include a summary of them here:

• Bounds and overflow checking
• Decidability by limitation of dynamic program behavior
• More precise gas estimation (as a consequence of decidability)
• Static typing
• Pure function support
• Binary fixed-point math
• Restrictions on reentrancy
• Static looping

The following is a list of goals or aspects of the existing Vyper project that we do not necessarily wish to duplicate:

• Lack of code re-use features such as:
  • Modifiers (re-usable assertions)
  • Method resolution and class inheritance (though we're on the fence about this)
  • Module imports
• Strict enforcement of limits on dynamic behavior -- Though we aim to provide Vyper's decidability by default, we believe it may also be useful to allow disabling of decidability features if the expressivity trade-off doesn't make sense for certain projects.

Additional goals

Stricter development practices

We wish to promote the following development practices:

• Thorough unit testing of all compiler components, from lexing and parsing routines to type checking and compilation routines.
• Thorough integration testing of combined components to identify issues with abstraction boundaries.
• Property-based testing or fuzz testing of components where appropriate. For examples of property-based testing frameworks, see QuickCheck for Rust, QuickCheck for Haskell, and hypothesis.
• The above testing goals should, at a minimum, give 100% code coverage but should also redundantly test the same code sections in different modes of use.
• Separation of concerns -- Components responsible for different stages of compilation (parsing, type checking, compilation, etc.) should be clearly separated into different libraries with distinct APIs.
• Maintainability -- Code should be written with future contributors in mind. Complex features should not only be automatically tested but also described in developer documentation. Code should be DRY and should include abstractions that naturally encapsulate specific tasks.

Support for eWASM and EVM and the use of YUL

We wish to leverage the YUL IR format as a means of targeting both the eWASM and EVM platforms. YUL is actively developed by the Solidity community and we see a great opportunity for synergy with the Solidity project through shared use of YUL.

Use of Rust as an implementation language

The Rust programming language has been a rising star in the PL community for a number of years. We believe it provides a reasonable balance between the following factors:

• Type and memory safety
• Ease of use
• Expressivity
• Size and quality of developer community
• Availability of modern language idioms such as iterators, functional programming features, etc.
• Support from established organizations such as Mozilla

The Rust project and Mozilla foundation are also some of the primary drivers behind the WASM project, which will likely end up playing a central role in the Ethereum community. The Rust compiler is one of the premier compilers to target the WASM platform.

The use of a compiled language in general, and Rust in particular, will facilitate improvements in the compiler release process. New compiler versions could be offered as statically compiled binaries (removing dependence on existing Python binaries) or as WASM binaries (opening up the possibility to run Fe in-browser or, in the distant future, on-chain).

Language specification and formal methods

We wish to provide a complete language specification with the following parts:

• Syntax specification in extended BNF format similar to the Python grammar specification.
• An operational, small-step semantics to describe correct program evaluation and behavior.
• Sections of informal prose to aid in understanding of the grammar, semantics, their relationship, and the context in which they operate.

A work in progress draft of the specification can be found here.

A secondary goal to providing a specification document could be to also provide an executable specification via tools such as the K framework. Providing such an executable specification may become a primary goal if it can be achieved without too much effort and without hampering the achievement of other goals.

Through the application of rigorous development practices and the availability of a complete language specification, we hope to apply formal verification methods to the Fe compiler and also to programs generated by it.

Progress and roadmap

Development of Fe is currently in its early stages. Here's a rough overview of recently completed work as well as some short and long-term goals. None of these lists should be considered exhaustive and should also not be considered to represent the exact prioritization of work items.

Recently achieved goals

• Completed port of python's stdlib tokenize module to Rust
• Settled on overall design of parser
• Completed a few basic parsers
• All code is now tested on both native and WASM platforms with WASM testing via node.js
• Achieve ~100% code coverage by tests. Coverage is currently maximized at ~99% due to some code paths being unreachable and our coverage tool currently having no support for ignoring specific lines of code.
• Identify an appropriate subset of the Python grammar and any grammatical extensions needed to define a Fe EBNF grammar
  • Eliminate certain Pythonic grammatical elements that don't apply to Fe (such as async constructs, lambda expressions, argument packing syntax like *args and **kwargs, etc.)
  • Include grammatical elements that more directly capture Fe language constructs (such as event, contract, or interface definitions, type dimensions, etc.)
• Finish writing parsers for the Fe EBNF grammar
• Parse a basic greeter contract to AST
• Compile the AST of a basic guest book contract to YUL
• Generate YUL source code from YUL AST objects (via yultsur)
• Invoke the Solidity compiler with generated YUL source to produce a compiled binary (via solc-rust)

Short-term goals

• Implement an ERC20 token contract in Fe and compile it.

Long-term goals

• Implement compilation routines for the full set of current Fe features
• Implement a YUL compiler in Rust to eliminate the need to interact with the Solidity compiler
• Formally verify the Fe compiler using the language specification

Community

• Twitter: @official_fe
• Chat: gitter/ethereum/fe