CλaSH is a functional hardware description language that borrows both its syntax and semantics from the functional programming language Haskell. Polymorphism and higherorder functions provide a level of abstraction and generality that allow a circuit designer to describe circuits in a more natural way than possible with the language elements found in the traditional hardware description languages.Circuit descriptions can be translated to synthesizable VHDL using the prototype CλaSH compiler. As the circuit descriptions, simulation code, and test input are also valid Haskell, complete simulations can be done by a Haskell compiler or interpreter, allowing high-speed simulation and analysis.

This paper introduces CλaSH, a novel hardware specification environment, by discussing several non-trivial examples. CλaSH is based on the functional language Haskell, and exploits many of its powerful abstraction mechanisms such as higher order functions, polymorphism, lambda abstraction, pattern matching, type derivation. As a result, specifications in CλaSH are concise and semantically clear, and simulations can be directly executed within a Haskell evaluation environment. CλaSH generates synthesizable low-level VHDL code by applying several transformation rules to a functional specification of a digital circuit.

This paper introduces the hardware specification system CλaSH by elaborating on a few non-trivial examples. CλaSH is a compiling system that translates a subset of Haskell into synthesizable VHDL by a rewriting technique. This subset of Haskell includes higher order functions, polymorphism, lambda abstraction, pattern matching, and choice constructs.

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Synchronous hardware can be straightforwardly modelled as a function from input and (current) state to an updated state and output. The CλaSH compiler can translate such a transition function, described in a functional language, to synthesisable VHDL. Taking a hardware-oriented viewpoint, components can then be seen as an instantiation of such a transition function. An abstraction called Arrows is used to directly model components by combining a transition function and its state. The abstraction also provides an uniform interface for composition, without losing the referential transparency offered by a functional description. Furthermore, readability of hardware designs is increased by the use of the γ-syntax, that automatically composes components according to the Arrow interface. The advantages of the Arrow abstraction and the γ-syntax are demonstrated by means of a realistic example circuit consisting of multiple components. This is a significant extension to CλaSH and enables many high level abstractions.