-\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
+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}[language=Clean,label={lst:exdeep},%
+\begin{lstlisting}[label={lst:exdeep},%
caption={A minimal deep \gls{EDSL}}]
:: DSL
= LitI Int
| Eq DSL
\end{lstlisting}
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}\cite{cheney_first-class_2003}. 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} and therefore the syntax is
-artificial. However, \glspl{GADT} 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.
+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 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.
-\begin{lstlisting}[language=Clean,label={lst:exdeepgadt},%
+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}[label={lst:exdeepgadt},%
caption={A minimal deep \gls{EDSL} using \glspl{GADT}}]
:: DSL a
= LitI Int -> DSL Int
| 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}[language=Clean,label={lst:exshallow},%
+\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
+Lit x = \e -> x
Var :: String -> DSL Int
-Var i = \e->retrEnv e i
+Var i = \e -> retrEnv e i
Plus :: (DSL Int) (DSL Int) -> DSL Int
-Plus x y = \e->x e + y e
+Plus x y = \e -> x e + y e
...
Eq :: (DSL a) (DSL a) -> DSL Bool | == a
-Eq x y = \e->x e + y e
+Eq x y = \e -> x e + y e
\end{lstlisting}
-The advantage of shallowly embedding a language in a host language is the
+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
-guarantee that the functionality is available. Moreover, the language is type
-safe as it is directly typed in the host language.
+of the host language guarantee whether 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 near
+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. This makes it slow for multiple views and complex to
+implement all views rendering 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
+advantages of both shallow and deep embedding. In class based shallow embedding
+the language constructs are defined as type classes. The same language is shown
with the new method in Listing~\ref{lst:exclassshallow}.
This type of embedding inherits the easiness of adding views from shallow
Just as with \glspl{GADT} in deep embedding type safety is guaranteed. Type
constraints are enforced through phantom types. One can add as many phantom
-type as is necessary. Lastly, extensions can be added easily, just as in
+type 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
+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}[language=Clean,label={lst:exclassshallow},%
+\begin{lstlisting}[label={lst:exclassshallow},%
caption={A minimal class based shallow \gls{EDSL}}]
:: Env = ... // Some environment
:: Evaluator a = Evaluator (Env -> a)
repositories / msc-thesis1617.git / blobdiff