-\section{\acrlong{EDSL}s}
There are several techniques available for creating \glspl{EDSL}. Each of
them have their own advantages and disadvantages such as extendability,
typedness and view support. In the following subsections each of the main
techniques are briefly explained.
-\subsection{Deep embedding}
+\section{Deep embedding}
A deep \gls{EDSL} means that the language is represented as an \gls{ADT}. Views
are functions that transform something to the datatype or the other way around.
As an example we have the simple arithmetic \gls{EDSL} shown in
-Listing~\ref{lst:exdeep}. Deep embedding has the advantage that it is very
-simple to build and the views are easy to make and add. However, there are also
-a few downsides.
+Listing~\ref{lst:exdeep}.
\begin{lstlisting}[language=Clean,label={lst:exdeep},%
caption={A minimal deep \gls{EDSL}}]
:: DSL
- = LitI Int
- | LitB Bool
- | Var String
- | Plus DSL DSL
+ = LitI Int
+ | LitB Bool
+ | Var String
+ | Plus DSL DSL
| Minus DSL DSL
- | And DSL DSL
- | Eq DSL
+ | And DSL DSL
+ | Eq DSL
\end{lstlisting}
-
-The expressions created with this language are not type-safe. In the given
-language it is possible an expression such as \CI{Plus (LitI 4) (LitB True)}
-which to add a boolean to an integer. Evermore so, extending the \gls{ADT} is
-easy and convenient but extending the views accordingly is tedious and has to
-be done individually for all views.
+Deep embedding has the advantage that it is very simple to build and the views
+are easy to make and add. However, there are also a few downsides. The
+expressions created with this language are not type-safe. In the given language
+it is possible an expression such as \CI{Plus (LitI 4) (LitB True)} which to
+add a boolean to an integer. Evermore so, extending the \gls{ADT} is easy and
+convenient but extending the views accordingly is tedious and has to be done
+individually for all views.
The first downside of the type of \gls{EDSL} can be overcome by using
-\glspl{GADT}. Listing~\ref{lst:exdeepgadt} shows the same language, but
-type-safe with a \gls{GADT}\footnote{\glspl{GADT} are not supported
-in the current version of \gls{Clean}. However, they can be simulated using
-bimaps}. Unfortunately the lack of extendability stays a problem. If a language
-construct is added no compile time guarantee is given that all views support
-it.
+\glspl{GADT}\cite{cheney_first-class_2003}. Listing~\ref{lst:exdeepgadt} shows
+the same language, but type-safe with a \gls{GADT}. \glspl{GADT} are not
+supported in the current version of \gls{Clean} and therefore the syntax is
+artificial. However, it has been shown that \glspl{GADT} can be simulated using
+bimaps or projection pairs\cite{cheney_lightweight_2002}. Unfortunately the
+lack of extendability stays a problem. If a language construct is added no
+compile time guarantee is given that all views support it.
\begin{lstlisting}[language=Clean,label={lst:exdeepgadt},%
caption={A minimal deep \gls{EDSL} using \glspl{GADT}}]
:: DSL a
- = LitI Int -> DSL Int
- | LitB Bool -> DSL Bool
- | Var String -> DSL Int
- | Plus (DSL Int) (DSL Int) -> DSL Int
- | Minus (DSL Int) (DSL Int) -> DSL Int
- | And (DSL Bool) (DSL Bool) -> DSL Bool
- | E.e: Eq (DSL e) (DSL e) -> DSL Bool & == e
+ = LitI Int -> DSL Int
+ | LitB Bool -> DSL Bool
+ | E.e: Var String -> DSL e
+ | Plus (DSL Int) (DSL Int) -> DSL Int
+ | Minus (DSL Int) (DSL Int) -> DSL Int
+ | And (DSL Bool) (DSL Bool) -> DSL Bool
+ | E.e: Eq (DSL e) (DSL e) -> DSL Bool & == e
\end{lstlisting}
-\subsection{Shallow embedding}
+\section{Shallow embedding}
In a shallowly \gls{EDSL} all language constructs are expressed as functions in
the host language. An evaluator view for our example language then looks
something like the code shown in Listing~\ref{lst:exshallow}. Note that much of
-the internals of the language can be hidden away using monads.
+the internals of the language can be hidden using monads.
\begin{lstlisting}[language=Clean,label={lst:exshallow},%
caption={A minimal shallow \gls{EDSL}}]
implement all views. This makes it slow for multiple views and complex to
implement.
-\subsection{Class based shallow embedding}
+\section{Class based shallow embedding}
The third type of embedding is called class based shallow embedding and has the
best of both shallow and deep embedding. In class based shallow embedding the
language constructs are defined as type classes. The same language is shown
:: PrettyPrinter a = PP String
class intArith where
- lit :: t -> v t | toString t
- add :: (v t) (v t) -> (v t) | + t
+ lit :: t -> v t | toString t
+ add :: (v t) (v t) -> (v t) | + t
minus :: (v t) (v t) -> (v t) | - t
class boolArith where
and :: (v Bool) (v Bool) -> (v Bool)
- eq :: (v t) (v t) -> (v Bool) | == t
+ eq :: (v t) (v t) -> (v Bool) | == t
instance intArith Evaluator where
lit x = \e->x
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