1 \chapter[chap:context]{Context}
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7 {\tfa Embedded domain-specific languages (\small{EDSL})}
11 \startcitedquotation[deursen00]
12 A domain-specific language (\small{DSL}) is a program-
13 ming language or executable specification language
14 that offers, through appropriate notations and ab-
15 stractions, expressive power focused on, and usu-
16 ally restricted to, a particular problem domain.
19 An embedded \small{DSL} is a \small{DSL} that is embedded in
20 another language. Haskell is commonly used to embed \small{DSL}s
21 in, which typically means a number of Haskell functions and types
22 are made available that can be called to construct a large value
23 of some domain-specific datatype (\eg, a circuit datatype). This
24 generated datatype can then be processed further by the
25 \small{EDSL} \quote{compiler} (which runs in the same environment
26 as the \small{EDSL} itself. The embedded language is then a, mostly
27 applicative, subset of Haskell where the library functions are the
28 primitives. Sometimes advanced Haskell features such as
29 polymorphism, higher-order values or type classes can be used in
30 the embedded language. \cite[hudak96]
32 For an \small{EDSL}, the definitions of compiletime and runtime
33 are typically fuzzy (and thus hard to define here), since the
34 \small{EDSL} \quote{compiler} is usually compiled by the same
35 Haskell compiler as the \small{EDSL} program itself.
39 An obvious question that arises when starting any research is \quote{Has
40 this not been done before?}. Using a functional language for describing hardware
41 is not a new idea at all. In fact, there has been research into functional
42 hardware description even before the conventional hardware description
43 languages were created. Examples of these are µFP \cite[sheeran83] and
44 Ruby \cite[jones90]. Functional languages were not nearly as advanced
45 as they are now, and functional hardware description never really got
48 Recently, there have been some renewed efforts, especially using the
49 Haskell functional language. Examples are Lava \cite[claessen00] (an
50 \small{EDSL}) and ForSyde \cite[sander04] (an \small{EDSL} using
51 Template Haskell). \cite[baaij09] has a more complete overview of the
54 We will now have a look at the existing hardware description languages,
55 both conventional and functional to see the fields in which Cλash is
58 \section{Conventional hardware description languages}
59 Considering that we already have some hardware description languages like
60 \small{VHDL} and Verilog, why would we need anything else? By introducing
61 the functional style to hardware description, we hope to obtain a hardware
62 description language that is:
64 \item More concise. Functional programs are known for their conciseness
65 and ability to provide reusable functions that are abstractions of
66 common patterns. This is largely enabled by features like an
67 advanced type system with polymorphism and higher-order functions.
68 \item Type-safer. Functional programs typically have a highly
69 expressive type system, which allows a programmer to more strictly
70 define limitations on values, making it easier to find violations
72 \item Easy to process. Functional languages have nice properties like
73 purity \refdef{purity} and single binding behaviour, which make it easy
74 to apply program transformations and optimizations and could potentially
75 simplify program verification.
78 \section[sec:context:fhdls]{Existing functional hardware description languages}
79 As noted above, this path has been explored before. However, current
80 embedded functional hardware description languages (in particular
81 those using Haskell) are limited. Below a number of downsides are
82 sketched of the recent attempts using the Haskell language.
83 \cite[baaij09] has a more complete overview of these and other
86 This list uses Lava and ForSyDe as examples, but tries to generalize
87 the conclusions to the techniques of embedding a \small{DSL} and
88 using Template Haskell.
91 \defref{Template Haskell}
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94 {\tfa Template Haskell}
98 Template Haskell is an extension to the \GHC\ compiler that allows
99 a program to mark some parts to be evaluated \emph{at compile
100 time}. These \emph{templates} can then access the abstract syntax
101 tree (\small{AST}) of the program that is being compiled and
102 generate parts of this \small{AST}.
104 Template Haskell is a very powerful, but also complex mechanism.
105 It was inteded to simplify the generation of some repetive pieces
106 of code, but its introspection features have inspired all sorts of
107 applications, such as hardware description compilers.
112 \item Not all of Haskell's constructs can be captured by embedded domain
113 specific languages. For example, an \hs{if} or \hs{case}
114 expression is typically executed once when the Haskell description
115 is processed and only the result is reflected in the generated
116 datastructure (and thus in the final program). In Lava, for
117 example, conditional assignment can only be described by using
118 explicit multiplexer components, not using any of Haskell's
119 compact mechanisms (such as \hs{case} expressions or pattern
122 Also, sharing of variables (\eg, using the same variable twice
123 while only calculating it once) and cycles in circuits are
124 non-trivial to properly and safely translate (though there is some
125 work to fix this, but that has not been possible in a completely
126 reliable way yet \cite[gill09]).
127 \item Template Haskell makes descriptions verbose. Template
128 Haskell needs extra annotations to move things around between
129 \small{TH} and the normal program. When \small{TH} is combined
130 with an \small{EDSL} approach, it can get confusing when to use
131 \small{TH} and when not to.
132 \item Function hierarchies cannot be observed in an \small{EDSL}.
133 For example, Lava generates a single flat \VHDL\ architecture,
134 which has no structure whatsoever. Even though this is strictly
135 correct, it offers no support to the synthesis software about
136 which parts of the system can best be placed together and makes
137 debugging the system very hard.
139 It is possible to add explicit annotation to overcome this
140 limitation (ForSyDe does this using Template Haskell), but this
141 adds extra verbosity again.
142 \item Processing in Template Haskell can become very complex,
143 since it works on the Haskell \small{AST} directly. This means
144 that every part of the Haskell syntax that is used must be
145 supported explicitly by the Template Haskell code.
146 \in{Chapter}[chap:prototype] shows that working on a smaller
147 \small{AST} is much more efficient.
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