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}
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}}]
:: 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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