Compiler Guide

Last updated 2 months ago

The Compiler Guide should quickly get you up to speed with the the codebase


The Flint compiler is separated into the following modules, all source files are contained within the Sources and each module has a directory of the same name that contains those files:

  • Source implements references to the Flint source code.

  • Diagnostic provides consistent errors, warnings and notes throughout the compiler for each of the different AST Passes.

  • Lexer decomposes the source code into tokens according to their contextual meaning. This is checked, in part, by Parser Tests.

  • Parser takes the lexer outputted tokens and consumes them to construct the abstract syntax tree. This is checked by Parser Tests.

  • AST contains anything related to the Abstract Syntax Tree (AST) and passes through it. Namely:

    • The nodes of the AST (ASTNode.swift, TopLevelModule.swift, Component/, Declaration/, Expression/, Statement/)

    • The AST framework for passes through the AST (ASTPass/, ASTVisitor/)

    • The environment which is used by each pass (Environment/)

    • The internal type system (Type.swift)

    • The dumper of the AST that is used for testing (ASTDumper.swift)

  • Semantic Analyzer is an AST Pass that checks that the valid syntax (source code that matches the grammar) is meaningful and outputs diagnostics relating to that. This is checked by Semantic Tests.

  • Type Checker is an AST Pass that checks if the types are correctly paired throughout the program. This is checked by Semantic Tests.

  • Optimizer is a stub.

  • IRGen ouputs the intermediate representation of the program (currently YUL (formerly IULIA) assembly code embedded in a Solidity contract). This comprises of:

    • A preprocessing AST pass that modifies the AST to strip out any convenience syntax and mangle function names so that there are no conflicts. (Preprocessor/)

    • IR generation structures for AST Nodes (IR*.swift) which render the IR strings.

  • Lite is the test runner.

  • File Check does a comparison of files. Used in Parser Tests.

  • flintc is the main module that controls the command line interface to pass to the Compiler (Compiler.swift) that coordinates all of the modules detailed above to generate the IR. It also preprends the source code of the Standard Library to the source code of the inputted files that is passed to the Solidity Compiler (SolcCompiler.swift).


Parser Tests (Tests/ParserTests)

The Parser Tests are a series Flint files which test the production of the Abstract Syntax Tree. Each parser test file is a .flint file which contains special comments of the form // CHECK-AST: XXX where XXX corresponds to a partial match of a line in the AST Dump of that program. Each file corresponds to a different aspect of the AST.

Semantic Tests

The Semantic Tests are a series of Flint files which test the diagnostic output of the compiler. Each semantic test file is a .flint file which contains special comments of the form // expected-XXX {{YYY}} where XXX corresponds to either a warning, error or optionally note and YYY corresponds to the diagnostic message that is outputted by the compiler. The semantic test files are compiled and the standard output is compared to these expected diagnostics according to the DiagnosticVerifier.swift file. Any unexpected diagnostics or the absence of an expected diagnostic leads to a test failure for that file.

Behavior Tests

The Behaviour Tests are the most comprehensive tests run using Truffle; they use the fully compiled bytecode on a test blockchain and check they perform as expected. Each one is contained in a separate folder in the tests/ subdirectory. They must contain:

  • A .flint program to compile and deploy to the blockchain

  • A test/config.js which specifies the contract name

  • A test/test/test.js file which details the actual tests run against that particular contract.

Either use the BehaviorTestTemplate or one of the prexisting tests such as array to get a better understanding of how these function.

You can also use the Truffle Reference for writing these tests.

Standard Library

The stdlib folder contains the Flint Standard Library which is special privileged flint code that can directly reference intermediate representation function calls and use $ in identifiers.

These provide core types and language functionality that can be expressed in Flint.


The docs folder contains any documentation for Flint, in particular the Flint Language Guide (, this Compiler Guide ( and the grammar in Augmented Backus-Naur Form (grammar.abnf). These should be updates along with any language features or compiler changes.


Contains all Flint Improvement Proposals (FIPs) which track the design and implementation of larger new features for Flint or the Flint compiler. An example is FIP-0001: Introduce the Asset trait.


The examples folder contains a few example Flint programs with basic functionality (valid/), solidity translations (solidity-translations/), some invalid code (invalid/), and future language examples (future/).


  • Snapshotting Code (

  • Vim integration (vim/)