Parser combinator in F# tuned to perform "well enough" on iOS (Xamarin)

Parsing.fs

module Parsing

/// Remember where we are in the code.
/// This is a struct to keep memory pressure down.
/// (Significant perf improvements on iOS.)
type ParseState =
    struct
        val Code : string
        val Index : int
        new (code : string, index : int) =
            { Code = code; Index = index }
    end
    member p.Rem = p.Code.Length - p.Index
    member p.WithIndex i = ParseState (p.Code, i)

/// Parsing results in either a value and the next parse state
/// or a failure (NoResult).
type ParseResult<'a> =
    | Value of 'a * ParseState
    | NoResult

/// A parser simply takes a parse state and produces a value
/// and the proceeding parse state.
type Parser<'a> = ParseState -> ParseResult<'a>

/// Stupid .NET number parsing functions can't work on substrings.
/// This is here just to keep the memory of lexing down.
let inline parseInt (s : string) (startIndex : int) (length : int) : int =
    let mutable r = 0
    let mutable i = 0
    while i < length do
        r <- (r * 10) + (int s.[startIndex + i] - int '0')
        i <- i + 1
    r

/// Lexing is slow mkay, and we don't want to slow it down
/// by constantly allocating strings. So we have this little
/// class to lazily get substrings from the source code.
type Substring =
    struct
        val Source : string
        val Index : int
        val Length : int
        new (source : string, index : int, length : int) =
            { Source = source; Index = index; Length = length }
    end
    member this.Value = if this.Length = 0 then "" else this.Source.Substring (this.Index, this.Length)
    member this.GetInt32Value () = parseInt this.Source this.Index this.Length
    member this.GetDoubleValue () =
        System.Double.Parse (this.Source.Substring (this.Index, this.Length),
                             System.Globalization.CultureInfo.InvariantCulture)
    member x.Eq (y : string) =
        let n = x.Length
        if n <> y.Length then false
        else
            let mutable ok = true
            let mutable i = 0
            while ok && i < n do
                ok <- x.Source.[x.Index + i] = y.[i]
                i <- i + 1
            ok
    override this.ToString () = this.Value

let emptySubstring = Substring ("", 0, 0)

/// Match two parsers in sequence.
let ( &&& ) (a : Parser<'a>) (b : Parser<'b>) : Parser<'a * 'b> =
    fun p ->
        match a p with
        | Value (av, ap) ->
            match b ap with
            | Value (bv, bp) -> Value ((av, bv), bp)
            | NoResult -> NoResult
        | NoResult -> NoResult

/// Match one or the other parsers.
let ( ||| ) (a : Parser<'a>) (b : Parser<'a>) : Parser<'a> =
    fun p ->
        match a p with
        | NoResult ->
            match b p with
            | NoResult -> NoResult
            | x -> x
        | x -> x

/// Transform the result of a parser. Use this to construct
/// your AST.
let ( >>> ) (a : Parser<'a>) (m : 'a -> 'b) : Parser<'b> =
    fun p ->
        match a p with
        | Value (av, ap) -> Value (m av, ap)
        | NoResult -> NoResult

/// Match two parsers in sequence and return the second value.
let ( &&> ) (a : Parser<'a>) (b : Parser<'b>) : Parser<'b> =
    fun p ->
        match a p with
        | Value (_, ap) ->
            match b ap with
            | Value (bv, bp) -> Value (bv, bp)
            | NoResult -> NoResult
        | NoResult -> NoResult

/// Match two parsers in sequence and return the first value.
let ( &&< ) (a : Parser<'a>) (b : Parser<'b>) : Parser<'a> =
    fun p ->
        match a p with
        | Value (av, ap) ->
            match b ap with
            | Value (_, bp) -> Value (av, bp)
            | NoResult -> NoResult
        | NoResult -> NoResult

/// Optional parsing - always succeeds
let opt (a : Parser<'a>) : Parser<'a option> =
    fun p ->
        match a p with
        | Value (av, ap) -> Value (Some av, ap)
        | NoResult -> Value (None, p)

/// Repeat a given parser 0 or more times
let zeroOrMore (a : Parser<'a>) : Parser<ResizeArray<'a>> =
    fun p ->
        let mutable cp = p
        let mutable cont = true
        let mutable r = ResizeArray<'a> ()
        while cont do
            match a cp with
            | Value (av, ap) -> r.Add (av); cp <- ap
            | NoResult -> cont <- false
        Value (r, cp)

/// Repeat a given parser 0 or more times with a given separator
let zeroOrMoreSep (sep) (a : Parser<'a>) : Parser<ResizeArray<'a>> =
    fun p ->
        let mutable cp = p
        let mutable cont = true
        let mutable r = ResizeArray<'a> ()
        let mutable needsSep = false
        while cont do
            let scp =
                if needsSep then
                    match sep cp with
                    | Value (_, p) -> p
                    | NoResult -> cont <- false; cp
                else cp
            if cont then
                match a scp with
                | Value (av, ap) -> r.Add (av); cp <- ap; needsSep <- true
                | NoResult -> cont <- false
        Value (r, cp)

/// Skip white space in a ParseState
let ws (s : ParseState) : ParseState =
    let n = s.Code.Length
    if s.Index >= n then s
    else
        let mutable i = s.Index
        let mutable ch = s.Code.[i]
        while i < n && System.Char.IsWhiteSpace (ch) do
            i <- i + 1
            if i < n then ch <- s.Code.[i]
        if i = s.Index then s
        else s.WithIndex i

/// A fixed sequence of characters
let tok (s : string) : Parser<string> =
    let n = s.Length
    fun p ->
        let p = ws p
        if p.Rem < n then NoResult
        else
            let mutable ok = true
            let mutable i = 0
            while ok && i < n do
                ok <- s.[i] = p.Code.[p.Index + i]
                i <- i + 1
            if ok then Value (s, p.WithIndex (p.Index + n))
            else NoResult

let isIdentStart (ch : char) =
    ch = '_' || System.Char.IsLetter (ch)
    
let isIdent (ch : char) =
    ch = '_' || System.Char.IsLetterOrDigit (ch)

/// A C-style identifier
let ident (p : ParseState) : ParseResult<Substring> =
    let p = ws p
    if p.Rem < 1 then NoResult
    else
        let mutable ok = isIdentStart p.Code.[p.Index]
        if not ok then NoResult
        else
            let n = p.Code.Length
            let start = p.Index
            let mutable i = start
            while ok && i < n do
                ok <- isIdent p.Code.[i]
                if ok then i <- i + 1
            let r = Substring (p.Code, start, i - start)
            Value (r, p.WithIndex i)

/// Parse a number with optional leading sign.
/// This is stuck on American style numbers.
let number (leadingSign : bool) (allowDecimal : bool) (p : ParseState) : ParseResult<Substring> =
    let p = ws p
    if p.Rem < 1 then NoResult
    else
        let start = p.Index
        let lead = p.Code.[p.Index]
        let mutable ok, i =
            match allowDecimal, leadingSign, lead with
            | _, true, '+' -> true, start + 1
            | _, true, '-' -> true, start + 1
            | true, _, '.' -> true, start
            | _, _, c when System.Char.IsDigit c -> true, start
            | _ -> false, start
        if not ok then NoResult
        else
            let n = p.Code.Length
            let mutable hasDig = false
            let mutable hasDecDot = false
            let mutable hasExp = false
            let mutable hasExpSign = false
            while ok && i < n do
                ok <- 
                    match p.Code.[i] with
                    | '.' -> if not allowDecimal || hasDecDot then false
                             else hasDecDot <- true; true
                    | 'e'
                    | 'E' -> if not allowDecimal || not hasDig || hasExp then false
                             else hasExp <- true; true
                    | '-' -> if not allowDecimal || not hasExp || hasExpSign then false
                             else hasExpSign <- true; true
                    | '+' -> if not allowDecimal || not hasExp || hasExpSign then false
                             else hasExpSign <- true; true
                    | c when System.Char.IsDigit c -> hasDig <- true; true
                    | _ -> false
                if ok then
                    i <- i + 1
            if hasDig then
                let r = Substring (p.Code, start, i - start)
                Value (r, p.WithIndex i)
            else NoResult

/// Parse a string delineated by the same start and stop char
/// While '\' is obeyed as in C, no post-processing is performed
/// to transform them.
let str (term : char) (p : ParseState) : ParseResult<Substring> =
    let p = ws p
    if p.Rem < 2 then NoResult
    else
        let mutable ok = (p.Code.[p.Index] = term)
        if not ok then NoResult
        else
            let n = p.Code.Length
            let start = p.Index
            let mutable i = start + 1
            while ok && i < n do
                if p.Code.[i] = term then ok <- false; i <- i + 1
                else
                    if p.Code.[i] = '\\' && (i + 1) < n then
                        i <- i + 2
                    else
                        i <- i + 1
            let r = Substring (p.Code, start + 1, i - start - 2)
            Value (r, p.WithIndex i)

/// Parse a regex and return the whole thing. You won't believe how
/// slow these functions are. Profile their usage, it's amazing.
let re (pattern : string) : Parser<string> =
    let r = new System.Text.RegularExpressions.Regex (pattern)
    fun p ->
        let m = r.Match (p.Code, p.Index)
        if m.Success && m.Index = p.Index then Value (m.Value, p.WithIndex (p.Index + m.Length))
        else NoResult

/// Parse a regex and return one of the groups.
/// This is great when prototyping a language, but omg,
/// don't use this in production. Regex is amazingly slow
/// and memory hungry.
let reg (pattern : string) (groupIndex : int) : Parser<string> =
    let r = new System.Text.RegularExpressions.Regex (pattern)
    fun p ->
        let m = r.Match (p.Code, p.Index)
        if m.Success && m.Index = p.Index then Value (m.Groups.[groupIndex].Value, p.WithIndex (p.Index + m.Length))
        else NoResult

@gregoryyoung Ah good point! I do all my work on iOS though and regex can't compile there.

@vasily-kirichenko FParsec doesn't work on iOS. Plus, isn't this cute? Pretty sure its faster too (FParsec does a lot more book keeping).