\chapter[chap:context]{Context}
- An obvious question that arises when starting any research is \quote{Hasn't
- this been done before?} Using a functional language for describing hardware
+ An obvious question that arises when starting any research is \quote{Has
+ this not been done before?} Using a functional language for describing hardware
is not a new idea at all. In fact, there has been research into functional
hardware description even before the conventional hardware description
- languages were created. However, functional languages were not nearly as
- advanced as they are now, and functional hardware description never really
- got off.
+ languages were created. \todo{Reference about early FHDLs} However,
+ functional languages were not nearly as advanced as they are now, and
+ functional hardware description never really got off.
+\todo{Add references}
Recently, there have been some renewed efforts, especially using the Haskell
- language. Examples are Lava, ForSyde, ..., which are all a form of an
+ functional language. Examples are Lava, ForSyde, ..., which are all a form of an
embedded domain specific language. Each of these have a slightly different
approach, but all of these do some trickery inside the Haskell language
itself, meaning you write a program that generates a hardware circuit,
instead of describing the circuit directly (either by running the haskell
code after compilation, or using Template Haskell to inspect parts of the
code you have written). This allows the full power of Haskell for generating
- a circuit, but only it also creates severe limitations in the use of the
+ a circuit. However it also creates severe limitations in the use of the
language (you can't use case statements in Lava, since they would be
executed only once during circuit generation) and extra notational overhead.
-TODO: Define (E)DSL
-TODO: References
+\fxnote{There should be a section on DSLs here}
- Advantages over conventional HDLs
- - More consise
- - More expressive
- - Easy to transform / optimize / etc.
+ We will now have a look at the existing hardware description languages,
+ both conventional and functional to see the fields in which Cλash is
+ operating.
- Advantages over existing FHDLs
- - More control
- - Full Haskell available
- - Concise notation
+ \section{Conventional hardware description languages}
+ Considering that we already have some hardware description languages like
+ \small{VHDL} and Verilog, why would we need anything else? By introducing
+ the functional style to hardware description, we hope to obtain a hardware
+ description language that is:
+ \startitemize
+ \item More consise. Functional programs are known for their conciseness
+ and ability to abstract away common patterns. This is largely enabled
+ by features like an advanced type system with polymorphism and higher
+ order functions.
+ \todo{Does this apply to FHDLs equally?}
+ \item Type-safer. Functional programs typically have a highly expressive
+ type system, which makes it harder to write incorrect code.
+ \item Easy to process. Functional languages have nice properties like
+ purity \refdef{purity} and single binding behaviour, which make it easy
+ to apply program transformations and optimizations and could potentially
+ simplify program verification.
+ \stopitemize
+
+ \section{Existing functional hardware description languages}
+ As noted above, we're not the first to walk this path. However, current
+ embedded functional hardware description languages (in particular those
+ using Haskell) are limited by:\todo{Separate TH and EDSL approaches
+ better}
+ \startitemize
+ \item Not all of Haskell's constructs can be captured by embedded domain
+ specific languages. For example, an if or case expression is typically
+ executed only once and only its result is reflected in the embedded
+ description, not the if or case expression itself. Also, sharing of
+ variables (\eg, using the same variable twice while only calculating it
+ once) and cycles in circuits are non-trivial to properly and safely
+ translate (though there is some work to fix this, but that has not been
+ possible in a completely reliable way yet. \todo{ref
+ http://www.ittc.ku.edu/~andygill/paper.php?label=DSLExtract09}
+ \item Some things are verbose to express. Especially ForSyDe suffers
+ from a lot of notational overhead due to the Template Haskell approach
+ used. Since conditional statements are not supported, a lot of Haskell's
+ syntax sugar (if expressions, pattern matching, guards) cannot be used
+ either, leading to more verbose notation as well.
+ \item Polymorphism and higher order values are not supported within the
+ embedded language. The use of Haskell as a host language allows the use
+ of polymorphism and higher order functions at circuit generation time
+ (even for free, without any additional cost on the \small{EDSL}
+ programmers), but the described circuits do not have any polymorphism
+ or higher order functions, which can be limiting. \todo{How true or
+ appropriate is this point?}
+ \todo[left]{Function structure gets lost (in Lava)}
+ \stopitemize
- Disadvantages over existing FHDLs
- - More work to implement advanced things
+ \todo[text]{Complete translation in TH is complex: Works with Haskell AST
+ instead of Core}
+
+% vim: set sw=2 sts=2 expandtab:
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