\begin{schemeregion}

Most aspects of the implementation of Typed Scheme are standard fare
for typed programming languages.  However, there is one key novelty:
the implementation is done entirely in terms of PLT Scheme macros.
This implementation choice provides the following advantages: 

\begin{itemize}
\item Running the program runs the typechecker---there is no separate
  checker to run, as there is with many static checkers for Scheme.
\item By expanding into untyped PLT Scheme code, integration with
  untyped code is seamless and requires no translation.
\item The typechecker integrates relatively smoothly with the rest of the macro
  and module system.
\item The typechecker can take advantage of existing PLT Scheme
  infrastructure. 
\end{itemize}

Implementing a typechecker as a macro provides its own
challenges.  In particular, the system must deal with the existence of
other macros, with cross-module interaction, and it must be able
to communicate information about the source program to the
typechecker, even though the expander is oblivious to the type system.  

The macro and module system in PLT Scheme is uniquely well-suited for
the implementation of Typed Scheme.  It contains a multitude of small
and large features, whose development has been guided by the goal of
supporting research on and development of new languages.  Even though
they may be of marginal use individually, together they form a
comprehensive language implementation framework.

This chapter first introduces the relevant prerequisites of the PLT Scheme macro
system (section~\ref{sect:macros}),
its integration with the module system
(section~\ref{sect:modules}), as well as how different macros can communicate
with each other (section~\ref{sect:protocols}); 
Typed Scheme's implementation uses all of these tools.
  The implementation is first described relative to a
single module (section~\ref{sect:type-one}), and then for the multi-module
case
(section~\ref{sect:type-multi}). 

% \section{Syntax Framework}
% \label{sect:syntax}

% The evolution of PLT Scheme has been guided by two major goals:
% \begin{itemize}
% \item implementing DrScheme and its teaching languages
% \item supporting research on programming tools and languages
% \end{itemize}
% These goals have pushed PLT Scheme to develop a powerful framework for
% representing and manipulating programs.

% This section presents a summary of the standard and nonstandard
% features of PLT Scheme's language framework.\footnote{While some of
% the features are PLT specific, many are supported in some form by
% other Scheme implementations.}
% %
% The purpose of this section is to show how these features are a part
% of a comprehensive language implementation framework, even those that
% appear to be of marginal interest when considered in isolation.

\input{syntax-macros}
\input{syntax-modules}

\subsection{Local expansion}

Some special forms must partially expand their bodies before
processing them. For example, primitive forms such as
\scheme{lambda} handle internal definitions by partially expanding
each form in the body to detect whether it is a definition or an
expression. The prefix of definitions is collected and transformed into a
\scheme{letrec} expression with the remainder of the original
forms in the body.

Macros can perform the same kind of partial expansion via the
\scheme{local-expand} procedure, which applies not just to expressions
but to entire modules as well.

\subsection{Compilation-unit hooks}
\label{sect:syntax:hooks}

There are two basic compilation scenarios in PLT Scheme. In
interactive mode, the compiler receives expressions from the
read-eval-print loop. In module mode, the compiler processes an entire
module at once. For each mode, the compiler provides a hook so the
macro system can be used to control compilation of that body of code.

\paragraph{Top-level transformers}

The top-level read-eval-print loop automatically wraps each interaction with the
\scheme{HPtop-interaction} macro. By defining a new version of the
\scheme{HPtop-interaction} macro, a programmer can customize the
behavior of each interaction.

\paragraph{Module transformers}

The macro expander processes a module from top to bottom,
partially expanding to uncover definitions, \scheme{require} and
\scheme{require-for-syntax} forms, and \scheme{provide} forms. It
executes syntax definitions and module import forms as it encounters
them. Then it performs another pass, expanding the remaining run-time
expressions.
%
The module system provides a hook, called \scheme{HPmodule-begin},
that allows language implementations to override the normal expansion
of modules.

The module transformer hook is typically used to constrain the
contents of the module or to automatically import modules into the
compile-time environment. For example, the \scheme{scheme} module
transformer inserts calls to print the values of all top-level
expressions in the module.

The module hook technique has been used before in language
experimentation. Specifically, \citet{pcmkf:continuations} prototyped a language for
programming web servlets using continuations. This prototype was the
first evidence that the module transformer is useful for
general-purpose language experimentation.
%
%FrTime also uses it to optimize functional reactive programs by
%combining nodes in the dataflow evaluation
%graph~\cite{burchett-frtime-opt}.

\input{syntax-protocols}


\end{schemeregion}
