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.

A straightforward synthesis from functional languages to digital circuits transforms variables to wires. The types of these variables determine the bit-width of the wires. Assigning a bit-width to polymorphic and function-type variables within this direct synthesis scheme is impossible. Using a term rewrite system, polymorphic and function-type binders can be completely eliminated from a circuit description, given only minor and reasonable restrictions on the input. The presented term rewrite system is used in the compiler for CλaSH: a polymorphic, higher-order, functional hardware description language.Functional Hardware Description Languages. SAFL [7] is a first-order hardware description language. As opposed to T C λ , which is used by CλaSH,

In order to effectively utilize the growing number of resources available on FPGAs, higher level abstraction mechanisms are needed to deal with increasing complexity resulting from large designs. Functional hardware description languages, like the CλaSH HDL, offer adequate abstraction mechanisms such as polymorphism and higher-order functions.This paper describes a two step design method to implement a DSP application on an FPGA, starting from a mathematical specification, followed by an implementation in CλaSH. A non trivial application, a particle filter, is used to evaluate both the method and CλaSH. First, a straightforward translation is performed from the mathematical definition of a particle filtering to Haskell, a functional programming language with syntax and semantics similar to CλaSH. Secondly, minor changes are applied to the Haskell implementation so that it is accepted by the CλaSH compiler. The resulting hardware produced by our method is evaluated and shows that this method eases reasoning about structure and parallelism in both the mathematical definition and the resulting hardware.

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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.