\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
  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. 

  Recently, there have been some renewed efforts, especially using the Haskell
  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
  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

  \section{Conventional hardware description languages}
    Considering that we already have some hardware description language 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,
      mostly caused by the ability to abstract just about any behaviour into a
      helper function. This is largely enabled by features like an advanced
      type system with polymorphism and higher order functions.
      \item Type-safer. Functional programs typically have a highly expressive
      type system, which makes it harder to write incorrect code. This is
      probably not only directly caused by the type system, so perhaps this
      advantage does not apply in hardware descriptions.
      \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:
    \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 and
      loops are non-trivial do 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?).
    \stopitemize
