\section{Introduction}
Hardware description languages has allowed the productivity of hardware
engineers to keep pace with the development of chip technology. Standard
-Hardware description languages, like \VHDL\ and Verilog, allowed an engineer
-to describe circuits using a programming language. These standard languages
-are very good at describing detailed hardware properties such as timing
-behavior, but are generally cumbersome in expressing higher-level
-abstractions. These languages also tend to have a complex syntax and a lack of
-formal semantics. To overcome these complexities, and raise the abstraction
-level, a great number of approaches based on functional languages has been
-proposed \cite{T-Ruby,Hydra,HML2,Hawk1,Lava,ForSyDe1,Wired,reFLect}. The idea
-of using functional languages started in the early 1980s \cite{Cardelli1981,
-muFP,DAISY,FHDL}, a time which also saw the birth of the currently popular
-hardware description languages such as \VHDL. What gives functional languages
-as hardware description languages their merits is the fact that basic
-combinatorial circuits are equivalent to mathematical function, and that
-functional languages lend themselves very well to describe and compose these
-mathematical functions.
+Hardware description languages, like \VHDL~\cite{VHDL2008} and
+Verilog~\cite{Verilog}, allowed an engineer to describe circuits using a
+programming language. These standard languages are very good at describing
+detailed hardware properties such as timing behavior, but are generally
+cumbersome in expressing higher-level abstractions. In an attempt to raise the
+abstraction level of the descriptions, a great number of approaches based on
+functional languages has been proposed \cite{T-Ruby,Hydra,HML2,Hawk1,Lava,
+ForSyDe1,Wired,reFLect}. The idea of using functional languages for hardware
+descriptions started in the early 1980s \cite{Cardelli1981, muFP,DAISY,FHDL},
+a time which also saw the birth of the currently popular hardware description
+languages such as \VHDL. The merit of using a functional language to describe
+hardware comes from the fact that basic combinatorial circuits are equivalent
+to mathematical functions and that functional languages are very good at
+describing and composing mathematical functions.
In an attempt to decrease the amount of work involved with creating all the
required tooling, such as parsers and type-checkers, many functional hardware
description languages are embedded as a domain specific language inside the
-functional language Haskell \cite{Hydra,Hawk1,Lava,ForSyDe1,Wired}. What this
-means is that a developer is given a library of Haskell functions and types
-that together form the language primitives of the domain specific language.
-Using these functions, the designer does not only describes a circuit, but
-actually builds a large domain-specific datatype which can be further
-processed by an embedded compiler. This compiler actually runs in the same
-environment as the description; as a result compile-time and run-time become
-hard to define, as the embedded compiler is usually compiled by the same
-Haskell compiler as the circuit description itself.
+functional language Haskell \cite{Hydra,Hawk1,Lava,ForSyDe1,Wired}. This
+means that a developer is given a library of Haskell~\cite{Haskell} functions
+and types that together form the language primitives of the domain specific
+language. As a result of how the signals are modeled and abstracted, the
+functions used to describe a circuit also build a large domain-specific
+datatype (hidden from the designer) which can be further processed by an
+embedded compiler. This compiler actually runs in the same environment as the
+description; as a result compile-time and run-time become hard to define, as
+the embedded compiler is usually compiled by the same Haskell compiler as the
+circuit description itself.
The approach taken in this research is not to make another domain specific
-language embedded in Haskell, but to use (a subset) of the Haskell language
-itself to be used as hardware description language. By taking this approach,
-we can capture certain language constructs, such as Haskell's choice elements
-(if-statement, case-statment, etc.), which are not available in the functional
-hardware description languages that are embedded in Haskell. As far as the
-authors know, such extensive support for choice-elements is new in the domain
-of functional hardware description language. As the hardware descriptions are
-plain Haskell functions, these descriptions can be compiled for simulation
-using using the optimizing Haskell compiler \GHC.
+language embedded in Haskell, but to use (a subset of) the Haskell language
+itself for the purpose of describing hardware. By taking this approach, we can
+capture certain language constructs, such as Haskell's choice elements
+(if-constructs, case-constructs, pattern matching, etc.), which are not
+available in the functional hardware description languages that are embedded
+in Haskell as a domain specific languages. As far as the authors know, such
+extensive support for choice-elements is new in the domain of functional
+hardware description language. As the hardware descriptions are plain Haskell
+functions, these descriptions can be compiled for simulation using using the
+optimizing Haskell compiler \GHC.
+
+Where descriptions in a conventional hardware description language have an
+explicit clock for the purpose state and synchronicity, the clock is implied
+in this research. The functions describe the behavior of the hardware between
+clock cycles, as such, only synchronous systems can be described. Many
+functional hardware description models signals as a stream of all values over
+time; state is then modeled as a delay on this stream of values. The approach
+taken in this research is to make the current state of the circuit part of the
+input of the function and the updated state part of the output of a function.
Like the standard hardware description languages, descriptions made in a
-functional hardware description languages must eventually be converted into a
-netlist. This research also features an a prototype translater called \CLaSH\
-(pronounced: Clash), which converts the Haskell code to equivalently behaving synthesizable \VHDL\ code, ready to be converted to an actual netlist format by optimizing \VHDL\ synthesis tools.
+functional hardware description language must eventually be converted into a
+netlist. This research also features a prototype translator called \CLaSH\
+(pronounced: clash), which converts the Haskell code to equivalently behaving
+synthesizable \VHDL\ code, ready to be converted to an actual netlist format
+by optimizing \VHDL\ synthesis tools.
\section{Hardware description in Haskell}
every \emph{type} used in a hardware description is needed.
The following types are \emph{built-in}, meaning that their hardware
- translation is fixed into the \CLaSH compiler. A designer can also
+ translation is fixed into the \CLaSH\ compiler. A designer can also
define his own types, which will be translated into hardware types
using translation rules that are discussed later on.
Haskell's builtin tuple types are also defined as single
constructor algebraic types and are translated according to this
- rule by the \CLaSH compiler. These types are translated to \VHDL\
+ rule by the \CLaSH\ compiler. These types are translated to \VHDL\
record types, with one field for every field in the constructor.
\item[\bf{No fields}]
Algebraic datatypes with multiple constructors, but without any
The merits of polymorphic typing, combined with higher-order functions, are
now also recognized in the `main-stream' hardware description languages,
-exemplified by the new \VHDL\-2008 standard~\cite{VHDL2008}. \VHDL-2008 has
+exemplified by the new \VHDL-2008 standard~\cite{VHDL2008}. \VHDL-2008 has
support to specify types as generics, thus allowing a developer to describe
polymorphic components. Note that those types still require an explicit
generic map, whereas type-inference and type-specialization are implicit in
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