From: Mart Lubbers <mart@martlubbers.net>
Date: Tue, 2 May 2017 17:57:22 +0000 (+0200)
Subject: Write bit about shallowly embedding
X-Git-Tag: hand-in~134
X-Git-Url: https://git.martlubbers.net/?a=commitdiff_plain;h=ca6e01e3ea526969a9cf4dfb48ee862f0c5e7544;p=msc-thesis1617.git

Write bit about shallowly embedding
---

diff --git a/methods.dsl.tex b/methods.dsl.tex
index 639eec4..c28ed62 100644
--- a/methods.dsl.tex
+++ b/methods.dsl.tex
@@ -12,8 +12,8 @@ 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}}]
+\begin{lstlisting}[language=Clean,label={lst:exdeep},%
+	caption={A minimal deep \gls{EDSL}}]
 :: DSL
 	= LitI Int
 	| LitB Bool
@@ -34,9 +34,10 @@ 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 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:exdeep},%
+\begin{lstlisting}[language=Clean,label={lst:exdeepgadt},%
 	caption={A minimal deep \gls{EDSL}}]
 :: DSL a
 	= LitI Int                     -> DSL Int
@@ -45,10 +46,46 @@ the lack of extendability stays a problem.
 	| 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
+	| 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. Our example language then looks something like the code
+shown in Listing~\ref{lst:exshallow} if implementing the evaluator. Note that
+much of the internals of the language can be hidden away 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.
 
 \subsection{Class based shallow embedding}
 \todo{while iTasks is also a DSL\ldots}