fsCheck sized generator from fare regex-based generator

Fare2FsCheck.fs

#r @"..\packages\Fare.1.0.0\lib\net35\Fare.dll"
#r @"..\packages\FsCheck.2.0.4\lib\net45\FsCheck.dll"

open Fare
open FsCheck
open System.Collections.Generic

// partial and naive implementation of a generator attempting to honor the size
let generator pattern =
    // pre-compute a certain amount of values and group them by size
    let xeger = Xeger pattern
    let dict = Dictionary<int, Queue<string>>()
    [for i in 1..1000 -> xeger.Generate()]
    |> Seq.groupBy String.length // use length for the size
    |> Seq.iter (fun (len, vals) -> dict.Add(len, vals |> Queue))

    dict |> Seq.iter (fun x -> printfn "size %i: %i items" x.Key x.Value.Count)

    // now build a sized generator as a lookup on the previously computed values
    fun size ->
        gen { return 
                match dict.TryGetValue size with
                | true, vals when vals.Count > 0 -> vals.Dequeue()
                // fallback to generating new values ignoring the size,
                // but a better strategy may be developed here, 
                // for example looking for values of similar size.
                // Another improvement needed is to somehow 'refill' dict 
                // once in a while, otherwise it soon gets empty
                | _ -> 
                    printfn "missing items of size %i" size
                    xeger.Generate() }
    |> Gen.sized
          

let example = generator "[abc]+"
for size in 1..10 do
    example
    |> Gen.sample size 5
    |> List.iter (printfn "size %i: %s" size)