1.1. Introduction to Clash

1.1.1. Functional Hardware

Clash is an open-source functional hardware description language (HDL) that closely mirrors the syntax and semantics of the Haskell programming language. It is used for creating hardware designs, typically for running on field programmable gate arrays (FPGAs) or application-specific integrated circuits (ASICs).

Clash is both a compiler, and a set of libraries for circuit design, that transform high level Haskell descriptions of synchronous, sequential logic into low-level VHDL, Verilog, or SystemVerilog. It provides a unique approach to design of sequential circuits, but with a high amount of abstraction power that blurs the line between strictly behavioral or structural synthesis approaches.

Clash aims to modernize the hardware development experience, making it easier to quickly and correctly develop complex circuit designs. This is achieved by making Clash:


Clash uses the Haskell type system to its full potential – including modern extensions and techniques – to being a high level of type safety and expressiveness to hardware design.

This expressive typing makes it easier to develop safe, maintainable hardware. Combinatorial and sequential logic is separated by type, and global safety invariants such as separating incompatible clock domains are enforced in the type system.


Clash makes it easy to express circuit designs in an intuitive manner, allowing high level structural components to be easily connected in designs. Moreover, unlike most “high level synthesis” tools, this extends to precise control over register placement and pipelining.


Unlike traditional HDL tools, Clash has a fully interactive read-eval-print loop (REPL), allowing circuits to be interactively designed and tested.


Clash reuses parts of the Glasgow Haskell Compiler to provide fast simulation of circuits for development and testing.


Clash uses a “whole program synthesis” approach in order to view the entire circuit at once, and optimizes this design before translating to a specific target. This allows meaningful optimizations to be performed on the entire design.


Additional primitives and black boxes can be added to Clash in the language of your choice, allowing you to use your own vendor or IP library within projects.

Clash allows seamless interoperability with libraries written in Haskell, including mtl, lens and QuickCheck. This makes it even easier to quickly prototype complex designs.

1.1.2. Intended Audience

Clash is ideal for developers from different backgrounds, although the main intended audiences are

Hardware Engineers

You are a hardware engineer, used to using tools like VHDL and Verilog to implement circuit designs. Clash offers the familar mixed simulation / synthesis capabilities of these tools, while providing a langauge with powerful abstractions.

Haskell Programmers

You are a Haskell programmer, looking to start developing hardware. Clash offers the ability to start prototyping and simulating designs in a familiar environment – lowering the learning curve significantly.

1.1.3. Maturity and Support

Clash is a continually evolving tool, having been actively developed since 2009. With the release of Clash 1.0 there has been an increased focus on maintaining API stability between releases, meaning circuit designs written in Clash should continue to work between minor releases. Today, the Clash Compiler is actively developed by QBayLogic B.V. and volunteers.

Several companies and enthusiasts are already using Clash to develop circuit designs, ranging from small designs on hobbyist boards to larger designs on modern FPGA and ASIC architectures.

While care is taken to thoroughly test the Clash compiler, some bugs may exist. We encourage users to file issues, or contribute pull requests on our GitHub repository.

1.1.4. Meta-information: Web Sites, Mailing Lists, etc.

Mailing list: for updates and questions join the mailing list clash-language+subscribe@googlegroups.com or read the forum

Slack: Invite yourself at fpchat-invite.herokuapp.com. To join #clash, click on “Channels” and search for “clash”.

IRC: freenode#clash-lang

1.1.5. Clash Version Numbering Policy

Clash follows the Haskell PVP Specification for its version numbers, for all packages. The main libraries that make up the Clash compiler maintain the same version numbers, making it easy to identify which versions are compatible.


Due to the Clash’s tight integration with GHC, updates to the GHC version that Clash uses result in changes to the Clash version. As GHC’s internals change frequently, even for minor bumps, it cannot be guaranteed that these changes will not result in Clash changes.

It is recommended (but not required) that downstream Clash packages and published Clash code also follow the PVP specification.