methods.dsl.tex

   1 \section{\acrlong{EDSL}s}
   2 There are several techniques available for creating \glspl{EDSL}. Each of
   3 them have their own advantages and disadvantages such as extendability,
   4 typedness and view support. In the following subsections each of the main
   5 techniques are briefly explained.
   6
   7 \subsection{Deep embedding}
   8 A deep \gls{EDSL} means that the language is represented as an \gls{ADT}. Views
   9 are functions that transform something to the datatype or the other way around.
  10 As an example we have the simple arithmetic \gls{EDSL} shown in
  11 Listing~\ref{lst:exdeep}. Deep embedding has the advantage that it is very
  12 simple to build and the views are easy to make and add. However, there are also
  13 a few downsides.
  14
  15 \begin{lstlisting}[language=Clean,label={lst:exdeep},caption={A minimal deep
  16 \gls{EDSL}}]
  17 :: DSL
  18         = LitI Int
  19         | LitB Bool
  20         | Var String
  21         | Plus DSL DSL
  22         | Minus DSL DSL
  23         | And DSL DSL
  24         | Eq DSL
  25 \end{lstlisting}
  26
  27 The expressions created with this language are not type-safe. In the given
  28 language it is possible an expression such as \CI{Plus (LitI 4) (LitB True)}
  29 which to add a boolean to an integer. Evermore so, extending the \gls{ADT} is
  30 easy and convenient but extending the views accordingly is tedious and has to
  31 be done individually for all views.
  32
  33 The first downside of the type of \gls{EDSL} can be overcome by using
  34 \glspl{GADT}. Listing~\ref{lst:exdeepgadt} shows the same language, but
  35 type-safe with a \gls{GADT}\footnote{Note that \glspl{GADT} are not supported
  36 in \gls{Clean}. They can be simulated using bimaps}\todo{cite}. Unfortunately
  37 the lack of extendability stays a problem.
  38
  39 \begin{lstlisting}[language=Clean,label={lst:exdeep},%
  40         caption={A minimal deep \gls{EDSL}}]
  41 :: DSL a
  42         = LitI Int                     -> DSL Int
  43         | LitB Bool                    -> DSL Bool
  44         | Var String                   -> DSL Int
  45         | Plus   (DSL Int) (DSL Int)   -> DSL Int
  46         | Minus  (DSL Int) (DSL Int)   -> DSL Int
  47         | And    (DSL Bool) (DSL Bool) -> DSL Bool
  48         | E.e: Eq (DSL e) (DSL e)       -> DSL Bool   & == e
  49 \end{lstlisting}
  50
  51 \subsection{Shallow embedding}
  52
  53 \subsection{Class based shallow embedding}
  54 \todo{while iTasks is also a DSL\ldots}
  55 \glspl{mTask} are expressed in a class based shallowly embedded \gls{EDSL}.
  56 There are two main types of \glspl{EDSL}.
  57 \todo{Small shallow embedded dsl intro}
  58 \todo{Small deep embedded dsl}
  59 \todo{Show that class based has the best of both worlds}