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-\section{\acrlong{EDSL}s}
-\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}
+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.
+
+\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}.
+
+\begin{lstlisting}[language=Clean,label={lst:exdeep},%
+	caption={A minimal deep \gls{EDSL}}]
+:: DSL
+	= LitI  Int
+	| LitB  Bool
+	| Var   String
+	| Plus  DSL DSL
+	| Minus DSL DSL
+	| And   DSL DSL
+	| 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}. \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
+	| 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}
+
+\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},%
+	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}
+
+The advantage of shallowly embedding a language in a host language is the
+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.
+
+The downside of this method is extending the language with views. It is near
+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.
+
+\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
+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} 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
+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}[language=Clean,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}