Question

So, I am writing code to parse and IP Address expression and turn it into a regular expression that could be run against and IP Address string and return a boolean response. I wrote the code in C# (OO) and it was 110 lines of code. I am trying to compare the amount of code and the expressiveness of C# to F# (I am a C# programmer and a noob at F#). I don’t want to post both the C# and F#, just because I don’t want to clutter the post. If needed, I will do so.

Anyway, I will give an example. Here is an expression:

192.168.0.250,244-248,108,51,7;127.0.0.1

I would like to take that and turn it into this regular expression:

((192\.168\.0\.(250|244|245|246|247|248|108|51|7))|(127\.0\.0\.1))

Here are some steps I am following:

Operations:

Break by “;” 192.168.0.250,244-248,108,51,7 127.0.0.1

Break by “.” 192 168 0 250,244-248,108,51,7

Break by “,” 250 244-248 108 51 7
Break by “-” 244 248

I came up with F# that produces the output. I am trying to forward-pipe through my operations listed above, as I think that would be more expressive. Can anyone make this code better? Teach me something 🙂

open System

let createItemArray (group:bool) (y:char) (items:string[]) = 
  [|
    let indexes = items.Length - 1
    let group = indexes > 0 && group
    if group then
      yield "("
    for i in 0 .. indexes do
      yield items.[i].ToString()
      if i < indexes then
        yield y.ToString()
    if group then
      yield ")"
  |] 

let breakBy (group:bool) (x:string) (y:char): string[] = 
  x.Split(y)
    |> createItemArray group y 

let breakItem  (x:string) (y:char): string[] = breakBy false x y
let breakGroup  (x:string) (y:char): string[] = breakBy true x y

let AddressExpression address:string = 
    let builder = new System.Text.StringBuilder "("
    breakGroup address ';'
    |> Array.collect (fun octet -> breakItem octet '.')
    |> Array.collect (fun options -> breakGroup options ',')
    |> Array.collect (fun (ranges : string) -> 
                            match (breakGroup ranges '-') with
                            | x when x.Length > 3
                              -> match (Int32.TryParse(x.[1]), Int32.TryParse(x.[3]))    with
                                  | ((true, a) ,(true, b))
                                      -> [|a .. b|]
                                          |> Array.map (int >> string)
                                          |> createItemArray false '-'
                                  | _ -> [|ranges|]
                            | _ -> [|ranges|]
                    )
    |> Array.iter (fun item ->
                    match item with
                    | ";" -> builder.Append ")|("
                    | "." -> builder.Append "\."
                    | "," | "-" -> builder.Append "|"
                    | _ -> builder.Append item
                    |> ignore
                  )
    builder.Append(")").ToString()

let address = "192.168.0.250,244-248,108,51,7;127.0.0.1"
AddressExpression address

Answer 1

Here’s mine in 63 lines of F# (including the one test case); it worked the first time, and feels pretty readable to me. It’s a typical parser-followed-by-pretty-printer. What do we think?

type IPs = IP[]
and IP = IP of OrParts * OrParts * OrParts * OrParts
and OrParts = Or of Part[]
and Part = Num of int | Range of int * int

let Valid(x) = if x < 0 || x > 255 then failwithf "Invalid number %d" x

let rec parseIPs (s:string) =
    s.Split [|';'|] |> Array.map parseIP
and parseIP s =
    let [|a;b;c;d|] = s.Split [|'.'|]
    IP(parseOrParts a, parseOrParts b, parseOrParts c, parseOrParts d)
and parseOrParts s =
    Or(s.Split [|','|] |> Array.map parsePart)
and parsePart s =
    if s.Contains("-") then
        let [|a;b|] = s.Split [|'-'|]
        let x,y = int a, int b
        Valid(x)
        Valid(y)
        if x > y then failwithf "Invalid range %d-%d" x y
        Range(x, y)
    else
        let x = int s
        Valid(x)
        Num(x)

let rec printIPsAsRegex ips =
    let sb = new System.Text.StringBuilder()
    let add s = sb.Append(s:string) |> ignore
    add "("
    add(System.String.Join("|", ips |> Array.map printIPAsRegex))
    add ")"
    sb.ToString()
and printIPAsRegex (IP(a, b, c, d)) : string =
    let sb = new System.Text.StringBuilder()
    let add s = sb.Append(s:string) |> ignore
    add "("
    printPartsAsRegex add a
    add "."
    printPartsAsRegex add b
    add "." 
    printPartsAsRegex add c
    add "."
    printPartsAsRegex add d
    add ")"
    sb.ToString()
and printPartsAsRegex add (Or(parts)) =
    match parts with
    | [| Num x |] -> // exactly one Num
        add(string x)
    | _ ->
        add "("
        add(System.String.Join("|", parts |> Array.collect (function
                | Num x -> [| x |]
                | Range(x,y) -> [| x..y |])
            |> Array.map (fun x -> x.ToString())))
        add ")"

let Main() =
    let ips = parseIPs "192.168.0.250,244-248,108,51,7;127.0.0.1"
    printfn "%s" (printIPsAsRegex ips)
Main()