# Announcing Pidgin v2.0

Errors, Span, and performance

Posted on January 26, 2019

Happy birthday to me! I’m pleased to announce that version 2.0 of my functional parsing library, Pidgin, is now available on Nuget. In this release I’ve focused on error messages, performance, and Span support.

## Parser Combinators

I haven’t written about Pidgin before, so allow me to briefly introduce it. Pidgin is a parser combinator library, meaning that it consists of three main concepts:

• A type Parser which models a parsing process
• A collection of primitive Parser objects which perform some simple individual parsing task
• A collection of combinator functions which can build complex Parsers out of simpler ones

Taken together, we have an object model allowing you to write code resembling a high-level description of a parsing process. As a brief taste, here is a simple example parser which parses an identifier in a typical programming language.

Parser<char, string> Identifier =
Letter.Then(
LetterOrDigit.ManyString(),
(first, rest) => first + rest
);

Assert.Equal("abc1", Identifier.ParseOrThrow("abc1"));
• Identifier is a Parser<char, string>, meaning it’s a process which consumes a sequence of chars and produces a string.
• Letter is a primitive parser which consumes and returns a single character from the input stream, moving it on to the next character. (If the character is not a letter, the parser fails and doesn’t change the state of the input stream.)
• LetterOrDigit is like Letter but for alphanumeric characters.
• ManyString is a combinator method which runs a parser in a loop until it fails. It takes all of the smaller parser’s results from the loop and packs them into a string. So LetterOrDigit.ManyString() is a parser which consumes and returns a sequence of alphanumeric characters.
• Then is another combinator which runs two parsers in sequence and applies a function to the result. So, reading the parser as a whole, we can see that an Identifier consists of a single letter followed by a sequence of letters or digits.

Parser combinators’ power comes from their composability. The library comprises a small number of building blocks, which you can put together in rich and varied ways to build a parser which does what you need. The library’s level of abstraction is a good fit for small-to-medium sized parsing tasks: it’s not as high-level as a full-blown parser generator like Antlr, but it’s much simpler to integrate. Rewriting our hand-written JQL parser in Pidgin took around four times less code.

That’s the overview — I won’t dive into a full tutorial on parser combinators because there are already plenty of those a mere Google away.

### How it works

Pidgin’s Parser<TToken, T> type represents a process which pulls TTokens one at a time from a (stateful) input stream, and either successfully returns a T or fails with an error message.

public abstract class Parser<TToken, T>
{
internal abstract Result<T> Parse(IParseState<TToken> input);
}
internal interface IParseState<TToken>
{
TToken? Peek();  // null if at end of file

// ... plus a few methods to facilitate backtracking etc
}
internal struct Result<T>
{
// if Success is true, Value contains the parsed value and Error is null
// if Success is false, Value is null and Error contains error info
public bool Success { get; }
public T Value { get; }
public ParseError Error { get; }
}

IParseState represents an iterator over the parser’s input (a sequence of TTokens). Peek returns the token at the current location in the input (or null if the parser has reached the end of the input) and Advance moves the stream on to the next token.

Primitive parsers typically manipulate the IParseState directly. This parser matches a specific string:

class StringParser : Parser<char, string>
{

internal override Result<string> Parse(IParseState<char> input)
{
foreach (var c in _expected)
{
var token = input.Peek();
if (!token.HasValue)
{
return Result.Error(ParseError.EndOfFile);
}
if (token.Value != c)
{
return Result.Error(ParseError.UnexpectedToken);
}
}
return Result.Success(_expected);
}
}

Higher-order parsers typically compose one or more smaller parsers, delegating to their Parse methods in some useful way. Then returns a parser which sequences two parsers and applies a mapping function to their results:

class ThenParser<TToken, T, U, R> : Parser<TToken, R>
{
private readonly Func<T, U, R> _resultSelector;

internal override Result<R> Parse(IParseState<TToken> input)
{
var result1 = _first.Parse(input);
if (!result1.Success)
{
return Result.Error(result1.Error);
}
var result2 = _second.Parse(input);
if (!result2.Success)
{
return Result.Error(result2.Error);
}
return Result.Success(_resultSelector(result1.Value, result2.Value));
}
}

## Error messages

Part of Pidgin’s job is to report when you gave a parser bad input. For example, the top level of a C# file must contain classes and namespaces, so feeding a C# parser the text Console.WriteLine("foo"); (without an enclosing class) should fail with some information about the problem:

Parse error.
unexpected 'C'
expected "class" or "namespace"
at line 1, col 1

A mistake I made early on in Pidgin’s development nearly two years ago(!) was trying to pre-compute the content of these error messages. Under the assumption that parsers are typically built once and then run repeatedly, I wrote some code to examine your parser upon construction and try to predict the ways it could fail on unexpected input, in order to avoid constructing error messages at runtime. This code calculated a set of “expected” input strings, accounting for Thens and Ors by concatenating the strings in the set and by unioning the sets respectively. (The idea was that a parser like Keyword("public").Optional().Then(Keyword("class")) would report that it expected "class" or "public class".)

This went catastrophically wrong for complex parsers. Here’s a sketch of some code which parses left-associative mathematical operators with precedence, so 3^2 + 4 * 3^5 * 5 is parsed as (3^2) + ((4 * (3^5)) * 5):

var topPrecedence = Number.Then(Char('^').Then(Number).Many());
var midPrecedence = topPrecedence.Then(Char('*').Then(topPrecedence).Many());
var lowPrecedence = midPrecedence.Then(Char('+').Then(midPrecedence).Many());

topPrecedence expects a number, or a number followed by a sequence of exponents — two possibilities. midPrecedence therefore expects a number, or a number followed by a sequence of exponents, or a number followed by a sequence of multiplications, or a number followed by a sequence of exponents followed by a sequence of multiplications — four possibilities. lowPrecedence has eight possible expected inputs. The set of expected inputs blows up exponentially! Calculating the error messages in advance means you need to explore all of the (exponentially large number of) expected inputs in advance, which led to cosmologically long parser build times. And in any case, reporting a huge number of expected inputs does not make for a very good error message.

I made some attempts to optimise this by using more efficient data structures, but I realised this was a losing proposition and decided to throw it out altogether in v2.0. Error messages are now computed at runtime, when the error actually occurs. This means I can be more precise about what the parser was expecting at that particular point in the input: once a parsing process has committed to a branch I can report expected inputs from that branch, rather than reporting all possible expected inputs.

Implementing this efficiently was a challenge. Parse errors actually occur quite frequently in a parser combinator library, even in the happy path, because of the way the prioritised choice operator Or works — String("foo").Or(String("bar")) only tries bar if an attempt to parse foo failed. So I tried to implement this change without allocating heap memory every time a parser fails. When a parser fails, it saves its expected inputs in a stack implemented on top of pooled memory (using ArrayPool), which are then popped if the error gets discarded. (The way that certain parsers manipulate their children’s error messages adds some interesting complications here, which I’ve described in a long comment.)

## Span

Note: You can watch my esteemed colleague Gervasio and me carrying out the work I describe here in a live-stream on YouTube. It was pretty fun!

I wanted to add support for parsing input stored in a Span. (Span is a new part of the BCL representing a reference to a contiguous block of memory such as a chunk of an array.) I already had functions which applied a Parser to a string, a T[], a Stream, etc; I’d abstracted over these various input types using the aforementioned IParseState interface.

internal interface IParseState<TToken>
{
TToken? Peek();
}

So adding Span support basically means implementing IParseState on top of a Span. In theory this should be quite straightforward — a class which has a Span and keeps track of its current position:

class SpanTokenStream<TToken> : ITokenStream<TToken>
{
private int _current;

TToken? Peek() => _current >= _span.Length ? null : _span[_current];

{
_current++;
}
}

But if you try and write this class, you’ll find that the compiler turns you away. Span is a ref struct, which means that it can only be stored on the stack. You can’t use a Span as a field of a class, or put it in an array, or box it (by upcasting it to an interface or object), or use it as a local variable in an async or yield method (because behind the scenes such methods copy their stack frame to the heap). (There are good reasons for this restriction, pertaining to memory safety.)

How to implement IParseState without storing a Span on the heap? Because IParseState is an internal interface, I can make certain guarantees about its usage. Instances of IParseState have a limited life-span — each IParseState instance becomes garbage before the call to Parse returns, and IParseState instances are never accessed from multiple threads. So I can store the Span in Parse’s stack frame and put a pointer to that stack frame in the IParseState.

This idea is complicated by the fact that Span is a managed type, so you’re not allowed to declare a Span<T>*. (This restriction ensures that a managed object can’t accidentally become garbage while it’s still being referred to by an unmanaged pointer; this needn’t trouble us here as the Span is guaranteed to be reachable because it’s on the stack.)

Fortunately, the System.Runtime.CompilerServices.Unsafe package contains some dangerous tools to get around this memory safety restriction: Unsafe.AsPointer coerces a ref to an untyped void* and Unsafe.AsRef coerces it back. So my plan was to store a ref Span<T> as a void* on the heap, and coerce it back to a ref Span<T> using when I need to address it as a Span. (This is a similar hack to one that I used in my HTML generation library.)

Unfortunately this doesn’t work either, because Unsafe.AsPointer is generic and you can’t use a ref struct as a type parameter! The compiler can’t be sure that Unsafe.AsPointer doesn’t box its argument, which of course is forbidden when the argument is a Span.

I wound up implementing a type-specialised copy of those two Unsafe methods:

static class Unsafe
{
public static void* AsPointer<T>(ref Span<T> span);
public static ref Span<T> AsRef(void* ptr);
}

AsPointer and AsRef are not expressible in C# (even unsafe C#); System.Runtime.CompilerServices.Unsafe is implemented in raw IL. My copies of those methods have stubs in C# which are filled in by rewriting the DLL’s IL in a post-compile step.

## Assorted Performance Improvements

Performance has always been one of Pidgin’s priorities — I’m proud that Pidgin is C#’s fastest parser combinator library (that I know of!) — but there’s always room for improvement. In my tests Pidgin still runs somewhat slower than F#’s FParsec library, for example. In this release I made some architectural changes to IParseState to help close that gap.

### Buffering Uniformly

As I mentioned earlier, Pidgin has several internal implementations of IParseState, each of which implements a streaming abstraction on top of a different type of input. Parsers may need to backtrack on failure (using the Try combinator), so you can’t always discard a token as soon as you’ve seen it. Some IParseState implementations — specifically the ones that are built on top of streaming storage (like Stream) — therefore buffer their input into an array. The ones that are backed by in-memory storage like string don’t need to buffer because their data is already in memory.

I decided to move the buffering logic to a shared part of the code. Now all IParseState implementations buffer their input, even the in-memory ones. On its own this should make the code slower (it’s more expensive to copy a string into an array than not to!), but it enables all of the optimisations I’m about to describe.

### De-Virtualisation & Inlining

Virtual method calls are comparatively expensive in .NET, and they happen frequently in Pidgin’s implementation because of IParseState’s design: a parser which consumes n tokens makes a minimum of n virtual calls to Advance.

With the buffering code extracted into a single place, there was no longer any need to keep it behind an interface. I moved all the buffering and error handling code into a mutable struct which is stored on the stack and passed to the Parsers by reference. Now Advance and Peek are non-virtual.

struct ParseState<TToken>
{
private TToken[] _buffer;
private int _currentPosition;
private ITokenStream<TToken> _stream;

public TToken? Peek() =>
_currentPosition < 0 || _currentPosition >= _buffer.Length
? null
: _buffer[_currentPosition];

{
_currentPosition++;

if (_currentPosition >= _buffer.Length)
{
}
}
}

public abstract class Parser<TToken, T>
{
internal abstract Result<T> Parse(ref ParseState<TToken> state);
}

That’s a fairly general C# performance trick: put shared state in a mutable struct, store it on the stack, and pass it around as a ref parameter. This both reduces GC pressure and improves the CPU cache’s hit rate, because the execution stack is likely to be in cache.

I also noticed that Peek was not inlining well. Inlining is an important optimisation carried out by the JIT compiler: if you call a method that’s short and simple enough, the compiler will just copy the method’s body into the current method instead of emitting code to call it. This can often enable further optimisations such as erasing array bounds checks.

I split Peek up into a pair of properties:

struct ParseState<TToken>
{
private TToken[] _buffer;
private int _currentPosition;

public bool HasCurrent => _currentPos >= 0 && _currentPos < _buffer.Length;
public TToken Current => _buffer[_currentPosition];
}

These properties are short and non-virtual, so they are good inlining candidates. After inlining them, the JIT sees simple array-manipulating code, which it can easily optimise in the context of the containing method. This simple change resulted in a 15-20% performance improvement across the board.

### Chunking

The Stream version of Advance called stream.Read(), which reads and returns a single byte from the stream. This is not as efficient as stream.Read(byte[]), which reads a chunk of bytes from the stream and copies them into an array. (The former requires n virtual method calls to read n bytes, whereas the latter requires only n / array.Length.)

I replaced ITokenStream’s Advance and Peek methods with a method which reads a chunk of tokens into the ParseState’s buffer.

interface ITokenStream<TToken>
{
void ReadInto(TToken[] buffer, int start, int count);
}

Why not design this signature as void ReadInto(Span<TToken> span)? I’d love to! Sadly that would preclude some implementations of ITokenStream. Methods like stream.Read(Span<byte> span) are available only on .NET Core, and as far as I’m aware Microsoft has no plans to backport them, so for compatibility with the desktop framework I’m stuck using arrays. This is irksome, as I’ve said before — what’s the point of designing a feature for library authors if you’re not going to support it properly?

ReadInto allows the ParseState to fill its buffer in chunks. But ParseState’s interface can also be chunk-ified — some Parsers (like String) can predict how many characters they’ll pull from the input. I added a Peek method to ParseState which returns a view into the ParseState’s buffer. (This makes Peek a little tricky to use correctly — you have to be careful not to continue using the Span after a call to Advance, which may mutate the buffer.)

public ReadOnlySpan<TToken> Peek(int count);

Using Peek, a parser like String("foo") can now look at three characters from the input to see if they match foo, rather than one at a time.

class StringParser : Parser<char, string>
{
internal override Result<string> Parse(ref ParseState<char> state)
{
var span = state.Peek(_expected.Length);
if (!_expected.AsSpan().SequenceEqual(span))
{
return Result.Failure();
}
return Result.Success(_expected);
}
}

This runs about 20% faster for long strings.