}
{\end{list}}
+\usepackage{paralist}
+
%include polycode.fmt
+%include clash.fmt
\begin{document}
%
\author{\IEEEauthorblockN{Christiaan P.R. Baaij, Matthijs Kooijman, Jan Kuper, Marco E.T. Gerards, Bert Molenkamp, Sabih H. Gerez}
\IEEEauthorblockA{University of Twente, Department of EEMCS\\
P.O. Box 217, 7500 AE, Enschede, The Netherlands\\
-c.p.r.baaij@utwente.nl, matthijs@stdin.nl}}
+c.p.r.baaij@@utwente.nl, matthijs@@stdin.nl}}
% \and
% \IEEEauthorblockN{Homer Simpson}
% \IEEEauthorblockA{Twentieth Century Fox\\
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.
+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.
+
+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.
+
+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.
-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.
\section{Hardware description in Haskell}
\subsection{Function application}
TODO: Pretty picture
- \subsection{Choices }
+ \subsection{Choices}
Although describing components and connections allows describing a
lot of hardware designs already, there is an obvious thing missing:
choice. We need some way to be able to choose between values based
expression, one using only case expressions and one using pattern
matching and guards.
-\begin{verbatim}
-sumif pred a b = if pred == Eq && a == b || pred == Neq && a != b
- then a + b
- else 0
-\end{verbatim}
+\begin{code}
+sumif pred a b =
+ if pred == Eq && a == b || pred == Neq && a != b
+ then a + b
+ else 0
-\begin{verbatim}
sumif pred a b = case pred of
- Eq -> case a == b of
- True -> a + b
- False -> 0
- Neq -> case a != b of
- True -> a + b
- False -> 0
-\end{verbatim}
-
-\begin{verbatim}
-sumif Eq a b | a == b = a + b
-sumif Neq a b | a != b = a + b
-sumif _ _ _ = 0
-\end{verbatim}
+ Eq -> case a == b of
+ True -> a + b
+ False -> 0
+ Neq -> case a != b of
+ True -> a + b
+ False -> 0
+
+sumif Eq a b | a == b = a + b
+sumif Neq a b | a != b = a + b
+sumif _ _ _ = 0
+\end{code}
TODO: Pretty picture
For algebraic types, we can make the following distinction:
\begin{xlist}
- \item[Product types]
+ \item[\textbf{Product types}]
A product type is an algebraic datatype with a single constructor with
two or more fields, denoted in practice like (a,b), (a,b,c), etc. This
is essentially a way to pack a few values together in a record-like
constructor algebraic data-types, including those with just one
field (which are technically not a product, but generate a VHDL
record for implementation simplicity).
- \item[Enumerated types]
+ \item[\textbf{Enumerated types}]
An enumerated type is an algebraic datatype with multiple constructors, but
none of them have fields. This is essentially a way to get an
enumeration-like type containing alternatives.
These types are translated to \VHDL\ enumerations, with one value for
each constructor. This allows references to these constructors to be
translated to the corresponding enumeration value.
- \item[Sum types]
+ \item[\textbf{Sum types}]
A sum type is an algebraic datatype with multiple constructors, where
the constructors have one or more fields. Technically, a type with
more than one field per constructor is a sum of products type, but
during a clock cycle. As we want to describe more than simple
combinatorial designs, \CLaSH\ needs an abstraction mechanism for state.
+ An important property in Haskell, and in most other functional languages,
+ is \emph{purity}. A function is said to be \emph{pure} if it satisfies two
+ conditions:
+ \begin{inparaenum}
+ \item given the same arguments twice, it should return the same value in
+ both cases, and
+ \item when the function is called, it should not have observable
+ side-effects.
+ \end{inparaenum}
+ This purity property is important for functional languages, since it
+ enables all kinds of mathematical reasoning that could not be guaranteed
+ correct for impure functions. Pure functions are as such a perfect match
+ for a combinatorial circuit, where the output solely depends on the
+ inputs. When a circuit has state however, it can no longer be simply
+ described by a pure function. Simply removing the purity property is not a
+ valid option, as the language would then lose many of it mathematical
+ properties. In an effort to include the concept of state in pure
+ functions, the current value of the state is made an argument of the
+ function; the updated state becomes part of the result.
+
+ A simple example is the description of an accumulator circuit:
+ \begin{code}
+ acc :: Word -> State Word -> (State Word, Word)
+ acc inp (State s) = (State s', outp)
+ where
+ outp = s + inp
+ s' = outp
+ \end{code}
+ This approach makes the state of a function very explicit: which variables
+ are part of the state is completely determined by the type signature. This
+ approach to state is well suited to be used in combination with the
+ existing code and language features, such as all the choice constructs, as
+ state values are just normal values.
\section{\CLaSH\ prototype}
foo\par bar
generic map, whereas type-inference and type-specialization are implicit in
\CLaSH.
-Wired~\cite{Wired},, T-Ruby~\cite{T-Ruby}, Hydra~\cite{Hydra}.
-
-A functional language designed specifically for hardware design is
-$re{\mathit{FL}}^{ect}$~\cite{reFLect}, which draws experience from earlier
-language called \acro{FL}~\cite{FL} to la
+% Wired~\cite{Wired},, T-Ruby~\cite{T-Ruby}, Hydra~\cite{Hydra}.
+%
+% A functional language designed specifically for hardware design is
+% $re{\mathit{FL}}^{ect}$~\cite{reFLect}, which draws experience from earlier
+% language called \acro{FL}~\cite{FL} to la
% An example of a floating figure using the graphicx package.
% Note that \label must occur AFTER (or within) \caption.
projects / matthijs / master-project / dsd-paper.git / blobdiff