Introduction

Clash is a functional hardware description language that borrows both its syntax and semantics from the functional programming language Haskell. It provides a familiar structural design approach to both combinational and synchronous sequential circuits.

Features of the Clash language:

• Strongly typed, but with a very high degree of type inference, enabling both safe and fast prototyping using concise descriptions.
• Interactive REPL: load your designs in an interpreter and easily test all your components without needing to set up a test bench.
• Higher-order functions, in combination with type inference, result in designs that are fully parametric by default.
• Synchronous sequential circuit design based on streams of values, called Signals, leads to natural descriptions of feedback loops.
• Multiple clock domains, with type-safe clock domain crossing.

Although we say that Clash borrows the semantics of Haskell, that statement should be taken with a grain of salt. What we mean to say is that the Clash compiler views a circuit description as a structural description. This means, in an academically handwavy way, that every function denotes a component and every function application denotes an instantiation of said component. Now, this has consequences on how we view recursively defined functions: structurally, a recursively defined function would denote an infinitely deeply structured component, something that cannot be turned into an actual circuit.

On the other hand, Haskell's by-default non-strict evaluation works very well for the simulation of feedback loops, which are ubiquitous in digital circuits. That is, when we take a structural view on circuit descriptions, value recursion corresponds directly to a feedback loop:

counter = s
 where
  s = register 0 (s + 1)

The above definition, which uses value recursion, can be synthesized to a circuit by the Clash compiler.

Over time, you will get a better feeling for the consequences of taking a structural view on circuit descriptions. What is always important to remember is that every applied function results in an instantiated component, and also that the compiler will never infer/invent more logic than what is specified in the circuit description.

With that out of the way, let us continue with installing Clash and building our first circuit.

Installing Clash

For installation instructions, see clash-lang.org/install/