-\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}
-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.
-
-\begin{lstlisting}[language=Clean,label={lst:exdeep},caption={A minimal deep
-\gls{EDSL}}]
+An \gls{EDSL} is a language embedded in a host language. \glspl{EDSL} can have
+one or more backends or views. Commonly used views are pretty printing,
+compiling, simulating, verifying and proving the program. There are several
+techniques available for creating \glspl{EDSL}. Each of them have their own
+advantages and disadvantages in terms of extendability, typedness and view
+support. In the following subsections each of the main techniques are briefly
+explained.
+
+\section{Deep embedding}
+A deep \gls{EDSL} is a language 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}.
+
+\begin{lstlisting}[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 add. To the downside, the expressions created with this language
+are not type-safe. In the given language it is possible to create an expression
+such as \CI{Plus (LitI 4) (LitB True)} that adds 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{Note that \glspl{GADT} are not supported
-in \gls{Clean}. They can be simulated using bimaps}\todo{cite}. Unfortunately
-the lack of extendability stays a problem.
+The first downside of this type of \gls{EDSL} can be overcome by using
+\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
+hypothetical. 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 remains 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:exdeep},%
- caption={A minimal deep \gls{EDSL}}]
+\begin{lstlisting}[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 using monads.
+
+\begin{lstlisting}[label={lst:exshallow},%
+ caption={A minimal shallow \gls{EDSL}}]
+:: Env = ... // Some environment
+:: DSL a = DSL (Env -> a)
+
+Lit :: a -> DSL a
+Lit x = \e -> x
+
+Var :: String -> DSL Int
+Var i = \e -> retrEnv e i
+
+Plus :: (DSL Int) (DSL Int) -> DSL Int
+Plus x y = \e -> x e + y e
+
+...
+
+Eq :: (DSL a) (DSL a) -> DSL Bool | == a
+Eq x y = \e -> x e + y e
+\end{lstlisting}
-\subsection{Class based shallow embedding}
-\todo{while iTasks is also a DSL\ldots}
-\glspl{mTask} are expressed in a class based shallowly embedded \gls{EDSL}.
-There are two main types of \glspl{EDSL}.
-\todo{Small shallow embedded dsl intro}
-\todo{Small deep embedded dsl}
-\todo{Show that class based has the best of both worlds}
+The advantage of shallowly embedding a language in a host language is its
+extendability. It is very easy to add functionality and compile time checks
+of the host language guarantee whether or not the functionality is available
+when used. Moreover, the language is type safe as it is directly typed in the
+host language.
+
+The downside of this method is extending the language with views. It is nearly
+impossible to add views to a shallowly embedded language. The only way of
+achieving this is by decorating the datatype for the \gls{EDSL} with all the
+information for all the views. This will mean that every component will have to
+implement all views rendering it slow for multiple views and complex to
+implement.
+
+\section{Class based shallow embedding}
+The third type of embedding is called class-based shallow embedding and has the
+advantages of both shallow and deep embedding. In class-based shallow embedding
+the language constructs are defined as type classes. This language is shown
+with the new method in Listing~\ref{lst:exclassshallow}.
+
+This type of embedding inherits the easiness of adding views from shallow
+embedding. A view is just a different data type implementing one or more of the
+type classes as shown in the aforementioned Listing where an evaluator and a
+pretty printer are implemented.
+
+Just as with \glspl{GADT}, type safety is guaranteed in deep embedding. Type
+constraints are enforced through phantom types. One can add as many phantom
+types as necessary. Lastly, extensions can be added easily, just as in
+shallow embedding. When an extension is made in an existing class, all views
+must be updated accordingly to prevent possible runtime errors. When an
+extension is added in a new class, this problem does not arise and views can
+choose to implement only parts of the collection of classes.
+
+\begin{lstlisting}[label={lst:exclassshallow},%
+ caption={A minimal class based shallow \gls{EDSL}}]
+:: Env = ... // Some environment
+:: Evaluator a = Evaluator (Env -> a)
+:: PrettyPrinter a = PP String
+
+class intArith where
+ 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
+
+instance intArith Evaluator where
+ lit x = \e->x
+ add x y = ...
+
+instance intArith PrettyPrinter where
+ lit x = toString x
+ add x y = x +++ "+" +++ y
+ ...
+\end{lstlisting}
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