source: doc/theses/andrew_beach_MMath/intro.tex @ d2cdd4f

\chapter{Introduction}

% The highest level overview of Cforall and EHMs. Get this done right away.
This thesis covers the design and implementation of the exception handling
mechanism (EHM) of
\CFA (pronounced sea-for-all and may be written Cforall or CFA).
\CFA is a new programming language that extends C, which maintains
backwards-compatibility while introducing modern programming features.
Adding exception handling to \CFA gives it new ways to handle errors and
make large control-flow jumps.

% Now take a step back and explain what exceptions are generally.
A language's EHM is a combination of language syntax and run-time
components that are used to construct, raise, and handle exceptions,
including all control flow.
Exceptions are an active mechanism for replacing passive error/return codes and return unions (Go and Rust).
Exception handling provides dynamic inter-function control flow.
There are two forms of exception handling covered in this thesis:
termination, which acts as a multi-level return,
and resumption, which is a dynamic function call.
% PAB: Maybe this sentence was suppose to be deleted?
Termination handling is much more common,
to the extent that it is often seen as the only form of handling.
% PAB: I like this sentence better than the next sentence.
% This separation is uncommon because termination exception handling is so
% much more common that it is often assumed.
% WHY: Mention other forms of continuation and \cite{CommonLisp} here?

Exception handling relies on the concept of nested functions to create handlers that deal with exceptions.
\begin{center}
\begin{tabular}[t]{ll}
\begin{lstlisting}[aboveskip=0pt,belowskip=0pt,language=CFA,{moredelim=**[is][\color{red}]{@}{@}}]
void f( void (*hp)() ) {
        hp();
}
void g( void (*hp)() ) {
        f( hp );
}
void h( int @i@, void (*hp)() ) {
        void @handler@() { // nested
                printf( "%d\n", @i@ );
        }
        if ( i == 1 ) hp = handler;
        if ( i > 0 ) h( i - 1, hp );
        else g( hp );
}
h( 2, 0 );
\end{lstlisting}
&
\raisebox{-0.5\totalheight}{\input{handler}}
\end{tabular}
\end{center}
The nested function @handler@ in the second stack frame is explicitly passed to function @f@.
When this handler is called in @f@, it uses the parameter @i@ in the second stack frame, which is accessible by an implicit lexical-link pointer.
Setting @hp@ in @h@ at different points in the recursion, results in invoking a different handler.
Exception handling extends this idea by eliminating explicit handler passing, and instead, performing a stack search for a handler that matches some criteria (conditional dynamic call), and calls the handler at the top of the stack.
It is the runtime search $O(N)$ that differentiates an EHM call (raise) from normal dynamic call $O(1)$ via a function or virtual-member pointer.

Termination exception handling searches the stack for a handler, unwinds the stack to the frame containing the matching handler, and calling the handler at the top of the stack.
\begin{center}
\input{termination}
\end{center}
Note, since the handler can reference variables in @h@, @h@ must remain on the stack for the handler call.
After the handler returns, control continues after the lexical location of the handler in @h@ (static return)~\cite[p.~108]{Tennent77}.
Unwinding allows recover to any previous
function on the stack, skipping any functions between it and the
function containing the matching handler.

Resumption exception handling searches the stack for a handler, does \emph{not} unwind the stack to the frame containing the matching handler, and calls the handler at the top of the stack.
\begin{center}
\input{resumption}
\end{center}
After the handler returns, control continues after the resume in @f@ (dynamic return).
Not unwinding allows fix up of the problem in @f@ by any previous function on the stack, without disrupting the current set of stack frames.

Although a powerful feature, exception handling tends to be complex to set up
and expensive to use
so it is often limited to unusual or ``exceptional" cases.
The classic example is error handling, where exceptions are used to
remove error handling logic from the main execution path, while paying
most of the cost only when the error actually occurs.

\section{Thesis Overview}
This work describes the design and implementation of the \CFA EHM.
The \CFA EHM implements all of the common exception features (or an
equivalent) found in most other EHMs and adds some features of its own.
The design of all the features had to be adapted to \CFA's feature set as
some of the underlying tools used to implement and express exception handling
in other languages are absent in \CFA.
Still the resulting basic syntax resembles that of other languages:
\begin{lstlisting}[language=CFA,{moredelim=**[is][\color{red}]{@}{@}}]
@try@ {
        ...
        T * object = malloc(request_size);
        if (!object) {
                @throw@ OutOfMemory{fixed_allocation, request_size};
        }
        ...
} @catch@ (OutOfMemory * error) {
        ...
}
\end{lstlisting}
% A note that yes, that was a very fast overview.
The design and implementation of all of \CFA's EHM's features are
described in detail throughout this thesis, whether they are a common feature
or one unique to \CFA.

% The current state of the project and what it contributes.
The majority of the \CFA EHM is implemented in \CFA, except for a small amount of assembler code.
In addition,
a suite of tests and performance benchmarks were created as part of this project.
The \CFA implementation techniques are generally applicable in other programming
languages and much of the design is as well.
Some parts of the EHM use features unique to \CFA, and hence,
are harder to replicate in other programming languages.
% Talk about other programming languages.
Three well known programming languages with EHMs, %/exception handling
C++, Java and Python are examined in the performance work. However, these languages focus on termination
exceptions, so there is no comparison with resumption.

The contributions of this work are:
\begin{enumerate}
\item Designing \CFA's exception handling mechanism, adapting designs from
other programming languages, and creating new features.
\item Implementing stack unwinding for the \CFA EHM, including updating
the \CFA compiler and run-time environment to generate and execute the EHM code.
\item Designing and implementing a prototype virtual system.
% I think the virtual system and per-call site default handlers are the only
% "new" features, everything else is a matter of implementation.
\item Creating tests and performance benchmarks to compare with EHM's in other languages.
\end{enumerate}

%\todo{I can't figure out a good lead-in to the roadmap.}
The thesis is organization as follows.
The next section and parts of \autoref{c:existing} cover existing EHMs.
New \CFA EHM features are introduced in \autoref{c:features},
covering their usage and design.
That is followed by the implementation of these features in
\autoref{c:implement}.
Performance results are presented in \autoref{c:performance}.
Summing up and possibilities for extending this project are discussed in \autoref{c:future}.

\section{Background}
\label{s:background}

Exception handling is a well examined area in programming languages,
with papers on the subject dating back the 70s~\cite{Goodenough75}.
Early exceptions were often treated as signals, which carried no information
except their identity. Ada~\cite{Ada} still uses this system.

The modern flag-ship for termination exceptions is \Cpp,
which added them in its first major wave of non-object-orientated features
in 1990.
% https://en.cppreference.com/w/cpp/language/history
While many EHMs have special exception types,
\Cpp has the ability to use any type as an exception.
However, this generality is not particularly useful, and has been pushed aside for classes, with a convention of inheriting from
\code{C++}{std::exception}.
While \Cpp has a special catch-all syntax @catch(...)@, there is no way to discriminate its exception type, so nothing can
be done with the caught value because nothing is known about it.
Instead the base exception-type \code{C++}{std::exception} is defined with common functionality (such as
the ability to print a message when the exception is raised but not caught) and all
exceptions have this functionality.
Having a root exception-type seems to be the standard now, as the guaranteed functionality is worth
any lost in flexibility from limiting exceptions types to classes.

Java~\cite{Java} was the next popular language to use exceptions.
Its exception system largely reflects that of \Cpp, except it requires
exceptions to be a subtype of \code{Java}{java.lang.Throwable}
and it uses checked exceptions.
Checked exceptions are part of a function's interface defining all exceptions it or its called functions raise.
Using this information, it is possible to statically verify if a handler exists for all raised exception, \ie no uncaught exceptions.
Making exception information explicit, improves clarity and
safety, but can slow down programming.
For example, programming complexity increases when dealing with high-order methods or an overly specified
throws clause. However some of the issues are more
programming annoyances, such as writing/updating many exception signatures after adding or remove calls.
Java programmers have developed multiple programming ``hacks'' to circumvent checked exceptions negating the robustness it is suppose to provide.
For example, the ``catch-and-ignore" pattern, where the handler is empty because the exception does not appear relevant to the programmer versus
repairing or recovering from the exception.

%\subsection
Resumption exceptions are less popular,
although resumption is as old as termination;
hence, few
programming languages have implemented them.
% http://bitsavers.informatik.uni-stuttgart.de/pdf/xerox/parc/techReports/
%   CSL-79-3_Mesa_Language_Manual_Version_5.0.pdf
Mesa~\cite{Mesa} is one programming languages that did. Experience with Mesa
is quoted as being one of the reasons resumptions are not
included in the \Cpp standard.
% https://en.wikipedia.org/wiki/Exception_handling
As a result, resumption has ignored in main-stream programming languages.
However, ``what goes around comes around'' and resumption is being revisited now (like user-level threading).
While rejecting resumption might have been the right decision in the past, there are decades
of developments in computer science that have changed the situation.
Some of these developments, such as functional programming's resumption
equivalent, algebraic effects\cite{Zhang19}, are enjoying significant success.
A complete reexamination of resumptions is beyond this thesis, but their re-emergence is
enough to try them in \CFA.
% Especially considering how much easier they are to implement than
% termination exceptions.

%\subsection
Functional languages tend to use other solutions for their primary EHM,
but exception-like constructs still appear.
Termination appears in error construct, which marks the result of an
expression as an error; thereafter, the result of any expression that tries to use it is also an
error, and so on until an appropriate handler is reached.
Resumption appears in algebraic effects, where a function dispatches its
side-effects to its caller for handling.

%\subsection
Some programming languages have moved to a restricted kind of EHM
called ``panic".
In Rust~\cite{Rust}, a panic is just a program level abort that may be implemented by
unwinding the stack like in termination exception handling.
% https://doc.rust-lang.org/std/panic/fn.catch_unwind.html
In Go~\cite{Go}, a panic is very similar to a termination, except it only supports
a catch-all by calling \code{Go}{recover()}, simplifying the interface at
the cost of flexibility.

%\subsection
While exception handling's most common use cases are in error handling,
here are other ways to handle errors with comparisons to exceptions.
\begin{itemize}
\item\emph{Error Codes}:
This pattern has a function return an enumeration (or just a set of fixed values) to indicate
if an error occurred and possibly which error it was.

Error codes mix exceptional and normal values, artificially enlarging the type and/or value range.
Some languages address this issue by returning multiple values or a tuple, separating the error code from the function result.
However, the main issue with error codes is forgetting to checking them,
which leads to an error being quietly and implicitly ignored.
Some new languages have tools that issue warnings, if the error code is
discarded to avoid this problem.
Checking error codes also results in bloating the main execution path, especially if an error is not dealt with locally and has to be cascaded down the call stack to a higher-level function..

\item\emph{Special Return with Global Store}:
Some functions only return a boolean indicating success or failure
and store the exact reason for the error in a fixed global location.
For example, many C routines return non-zero or -1, indicating success or failure,
and write error details into the C standard variable @errno@.

This approach avoids the multiple results issue encountered with straight error codes
but otherwise has many (if not more) of the disadvantages.
For example, everything that uses the global location must agree on all possible errors and global variable are unsafe with concurrency.

\item\emph{Return Union}:
This pattern replaces error codes with a tagged union.
Success is one tag and the errors are another.
It is also possible to make each possible error its own tag and carry its own
additional information, but the two branch format is easy to make generic
so that one type can be used everywhere in error handling code.

This pattern is very popular in functional or any semi-functional language with
primitive support for tagged unions (or algebraic data types).
% We need listing Rust/rust to format code snipits from it.
% Rust's \code{rust}{Result<T, E>}
The main advantage is providing for more information about an
error, other than one of a fix-set of ids.
While some languages use checked union access to force error-code checking,
it is still possible to bypass the checking.
The main disadvantage is again significant error code on the main execution path and cascading through called functions.

\item\emph{Handler Functions}:
This pattern implicitly associates functions with errors.
On error, the function that produced the error implicitly calls another function to
handle it.
The handler function can be provided locally (passed in as an argument,
either directly as as a field of a structure/object) or globally (a global
variable).
C++ uses this approach as its fallback system if exception handling fails, \eg
\snake{std::terminate_handler} and for a time \snake{std::unexpected_handler}

Handler functions work a lot like resumption exceptions, without the dynamic handler search.
Therefore, setting setting up the handler can be more complex/expensive, especially if the handle must be passed through multiple function calls, but cheaper to call $O(1)$, and hence,
are more suited to frequent exceptional situations.
% The exception being global handlers if they are rarely change as the time
% in both cases shrinks towards zero.
\end{itemize}

%\subsection
Because of their cost, exceptions are rarely used for hot paths of execution.
Therefore, there is an element of self-fulfilling prophecy for implementation
techniques to make exceptions cheap to set-up at the cost
of expensive usage.
This cost differential is less important in higher-level scripting languages, where use of exceptions for other tasks is more common.
An iconic example is Python's @StopIteration@ exception that is thrown by
an iterator to indicate that it is exhausted, especially when combined with Python's heavy
use of the iterator-based for-loop.
% https://docs.python.org/3/library/exceptions.html#StopIteration