Parsers combinators with F#6

README.md

Parsers combinators with F#6 [<InlineIfLambda>]

Thanks to manofstick peer reviewing the blog post.

Full source code at github

For F# Advent 2021 I wrote a blog post exploring how F#6 [<InlineIfLambda>] can improve data pipeline performance.

I was thinking of other places where [<InlineIfLambda>] can help and decided to try to build a parser combinator library with [<InlineIfLambda>].

Parser combinators

FParsec is a parser combinator library that allows you to create complex parsers by combining simple parsers. FParsec is a great library and I was amazed by it the first time I tried it.

Graham Hutton and Erik Meijer have written an excellent introduction to parser combinators. When I read this article many years ago it was the first time that I felt that I began to understand what Functional Programming is all about.

We create a parser combinator library by defining what is a parser and then define various functions that combine parsers.

A Parser could be this:

type 'T Parser = string -> ('T*string) option

This function takes a string, produces a value and the unconsumed string. If the parser fails it returns None.

It's possible to build a parser combinator library around this definition but it won't be efficient as we will constantly create substrings.

FParser

My first attempt was this:

type 'T Parser = string -> int -> ('T*int) option

The parser takes a string and the current position and returns a value and the position of the first unconsumed char if successful.

While experimenting with performance I found that the overhead from creating tuples and options was significant, switching to value tuples and options reduced performance but avoided GC pressure.

In the end I ended up with this:

type FParserContext(input : string) =
  class
    [<DefaultValue>] val mutable Pos : int

    member x.Input = input
  end

type 'T FParser = FParserContext -> 'T

The ParserContext stores the string to be parsed and the current position. A parser takes a ParserContext and returns a value if successful and updates the position in the context. Upon failure, the parser returns the default value of 'T and has set the position to a negative value.

Not elegant but it was the most performant option I found. There might be better ones.

In addition; there are a number of parser combinators defined and I won't go through all of them but I will show how <|> works which is the choice combinator.

  // <|> runs the first parser and if successful returns its value
  //  , otherwise runs the second
  // Tag as inline + InlineIfLambda to inline both the function and the nested parsers
  let inline (<|>) ([<InlineIfLambda>] pf : _ FParser) ([<InlineIfLambda>] ps : _ FParser) : _ FParser =
    fun c ->
      // Save the current pos
      let spos = c.Pos
      // Runs the first parser
      let fv = pf c
      // Did we succeed?
      if c.IsGood () then
        // Yes
        success fv
      else
        // Otherwise restore the position and run the second parser
        c.Pos <- spos
        ps c

The other combinators are implemented in similar style.

With the parser combinator library one can implement a simple math expression parser

// Expression tree for math expressions like: x*(2+123*y)
type Expr =
  | Value     of int
  | Variable  of string
  | Binary    of char*(int->int->int)*Expr*Expr

module FParserCalculator =
  open FParser.Core
  open FParser

  let inline ptoken label v =
    pskipChar label v
    >>. pskipWhitespace ()

  let inline pop exp ([<InlineIfLambda>] t) ([<InlineIfLambda>] m) =
    pcharSat exp t |>> m .>> pskipWhitespace ()

  // Normally should have been a module but that interacts poorly with
  //  for some reason Benchmark.NET
  type ParserConfig () =
    class
      // Since the grammar is recursive use pfwd to do a forward declaration of
      //  pexpr
      let struct (pexpr, sexpr) = pfwd<Expr> ()

      // A term is either
      let pterm : Expr FParser =
            // An integer
            (pint () |>> Value .>> pskipWhitespace ())
            // A variable
        <|> (pstringSat1 "variable" (fun ch p -> (ch >= 'A' && ch <= 'Z') || (ch >= 'a' && ch <= 'z')) |>> Variable .>> pskipWhitespace ())
            // Or a sub expression wrapped in parantheses
        <|> (ptoken "(" '(' >>. pexpr .>> ptoken ")" ')')

      let op0 ch =
        match ch with
        | '*' -> fun l r -> Binary ('*', ( * ), l, r)
        | '/' -> fun l r -> Binary ('/', ( / ), l, r)
        | _   -> failwith "Expected * or /"
      let op1 ch =
        match ch with
        | '+' -> fun l r -> Binary ('+', ( + ), l, r)
        | '-' -> fun l r -> Binary ('-', ( - ), l, r)
        | _   -> failwith "Expected + or -"
      // pop0 parses expression separated by */
      let pop0 : Expr FParser =
        pterm >+> pchainLeft1 (pop "*/" (fun ch -> ch = '*' || ch ='/') (fun ch -> op0 ch))
      // pop1 parses expression separated by +-
      //  by splitting it in this way we get different precendece for */ and +-
      let pop1 : Expr FParser =
        pop0 >+> pchainLeft1 (pop "+-" (fun ch -> ch = '+' || ch ='-') (fun ch -> op1 ch))
      do
        // pop1 is the complete the forward declared pexpr
        //  set pexpr to pop1
        sexpr pop1
      // The full grammar
      let pfull = pskipWhitespace () >>. pop1 .>> peof ()

      member x.Parse s = prun pfull s
    end

I also implemented a math expression parser using FParsec and painstakingly implemented a Baseline math expression parser in imperative style.

Benchmark results

Using Benchmark.net I compared the performance of FParser, FParsec and Baseline. As a late bonus I aslo implemented a calculator expression parser in C# using a delegate based parser combinator library.

BenchmarkDotNet=v0.13.1, OS=Windows 10.0.19044.1415 (21H2)
Intel Core i5-3570K CPU 3.40GHz (Ivy Bridge), 1 CPU, 4 logical and 4 physical cores
.NET SDK=6.0.101
  [Host] : .NET 6.0.1 (6.0.121.56705), X64 RyuJIT DEBUG
  PGO    : .NET 6.0.1 (6.0.121.56705), X64 RyuJIT
  STD    : .NET 6.0.1 (6.0.121.56705), X64 RyuJIT


|                     Method | Job |       Mean |    Error |   StdDev |  Gen 0 | Allocated |
|--------------------------- |---- |-----------:|---------:|---------:|-------:|----------:|
|   Baseline_BasicExpression | STD |   197.9 ns |  1.63 ns |  1.44 ns | 0.0610 |     192 B |
|    FParser_BasicExpression | STD |   245.2 ns |  1.40 ns |  1.31 ns | 0.0710 |     224 B |
|    FParsec_BasicExpression | STD |   734.3 ns |  4.70 ns |  4.17 ns | 0.1631 |     512 B |
|   CsParser_BasicExpression | STD |   514.5 ns |  3.54 ns |  3.31 ns | 0.0610 |     192 B |
| Baseline_ComplexExpression | STD |   579.4 ns |  2.69 ns |  2.38 ns | 0.2699 |     848 B |
|  FParser_ComplexExpression | STD |   721.9 ns | 10.04 ns |  9.39 ns | 0.2804 |     880 B |
|  FParsec_ComplexExpression | STD | 1,731.1 ns |  7.95 ns |  6.64 ns | 0.3986 |   1,256 B |
| CsParser_ComplexExpression | STD | 1,375.4 ns |  6.31 ns |  5.91 ns | 0.2689 |     848 B |
|   Baseline_BasicExpression | PGO |   171.5 ns |  1.68 ns |  1.57 ns | 0.0610 |     192 B |
|    FParser_BasicExpression | PGO |   222.5 ns |  2.39 ns |  2.23 ns | 0.0713 |     224 B |
|    FParsec_BasicExpression | PGO |   617.4 ns |  5.82 ns |  5.45 ns | 0.1631 |     512 B |
|   CsParser_BasicExpression | PGO |   425.6 ns |  6.12 ns |  5.72 ns | 0.0610 |     192 B |
| Baseline_ComplexExpression | PGO |   449.1 ns |  2.06 ns |  1.93 ns | 0.2699 |     848 B |
|  FParser_ComplexExpression | PGO |   615.8 ns |  8.99 ns |  8.41 ns | 0.2804 |     880 B |
|  FParsec_ComplexExpression | PGO | 1,564.1 ns | 14.88 ns | 13.92 ns | 0.3986 |   1,256 B |
| CsParser_ComplexExpression | PGO | 1,228.4 ns |  6.07 ns |  5.68 ns | 0.2689 |     848 B |

This does look very promising. As expected the Baseline parser does the best but FParser is not far behind. FParsec has very respectable performance but FParser manages to do a bit better. Now FParsec supports great error messages which is currently not implemented in FParser but there are ways to support error messages without too much overhead.

Another nice aspect of FParser is that it adds less memory overhead than FParsec, the overhead comes from creating FParserContext

CsParser falls between FParser and FParsec which is pretty good considering it's delegate based. CsParser also don't collect error information so comparing to FParsec is a bit unfair here as well.

PGO uses a new feature in the runtime called Dynamic Profile-Guided Optimization which allows the jitter to improve code over time and does seem to give benefit to all parser variants over the standard approach.

Conclusion

There is much more to investigate and improve but once again [<InlineIfLambda>] seems like its a powerful tool that allows us to write functional style programs with close to imperative performance.

Regards,

Mårten

Scott Wlaschin also has an excellent talk on parsers that really help me grok the subject, highly recommended - https://fsharpforfunandprofit.com/parser/

@davidglassborow thanks!