FsGenAll - Generate all possible values for an F# type

0_README.md

FsGenAll - Generate all possible values for an F# type

writing test code I often use FsCheck. However, sometimes I want to generate each possible value for a test model and check the properties for that.

My typical use case is something like this:

1. Create an ADT that models what I want to test
2. Generate all possible values of this test model
3. Generate test data from the test model
4. Run the function under test that produces a result
5. Compare the result to the test model
6. Pass if in agreement

For example I could want to test a JSON parser and while FsCheck is great most strings it produces aren't JSON.

To complement the rest of my test suite I could define a test model like this:

type ArrayValue<'T> =
| Empty
| One of 'T
| Two of 'T*'T

type NumberValue =
| Zero
| One
| PI

type StringValue =
| Empty
| Hello
| Escaped

type JsonValue =
  | NullValue
  | StringValue   of StringValue
  | NumberValue   of NumberValue
  | BooleanValue  of bool
  | ArrayValue    of JsonValue ArrayValue
  | ObjectValue   of (StringValue*JsonValue) ArrayValue

Then using FsCheck I generate a bunch of values and then check the JSON parser properties. But FsCheck tends to generate fair amount of duplicates and sometimes I want to test the entire set of possible values.

That's why I created FsGenAll.

let jsonValueGen  = FsGen.genAll<JsonValue> 6
let genAll ()     = jsonValueGen |>> toArray

Calling genAll gives me all possible values of JsonValue to a maximum depth of 6.

On my aging machine it FsGenAll a performance estimate looks like this:

Number of values: 25751
Took 33 ms

Turning of JIT tail calls improves performance by about 50%:

Number of values: 25751
Took 22 ms

JIT tail calls has been much improved in .NET6 but it does add a bit overhead most of the times.

Update

Updated code pattenr and code seems to do better and tailcall and no tailcall seems perform similarily.

1_FsGenAll.fs

(*
Copyright © 2022 Mårten Rånge

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the “Software”), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell 
copies of the Software, and to permit persons to whom the Software is furnished
to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED “AS IS”, WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER 
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.
*)

// I wrote a blog about PushStream for F# advent 2022
//  https://github.com/mrange/PushStream6
//  I am appealed to them both because of their simplicity and performance
//  When generating values I need a place to my values after creation
//  PushStreams turn out to be very convenient
type 'T PushStream = ('T -> bool) -> bool

module PushStream =
  [<GeneralizableValue>]
  let empty<'T> : 'T PushStream = fun r -> true

  let inline map ([<InlineIfLambda>] m) ([<InlineIfLambda>] ps : 'T PushStream) : 'U PushStream =
    fun r ->
      ps (fun v -> r (m v))

  let inline toResizeArray (capacity : int) ([<InlineIfLambda>] ps : 'T PushStream) : _ ResizeArray =
    let ra = ResizeArray capacity
    ps (fun v -> ra.Add v; true) |> ignore
    ra

  let inline toArray ([<InlineIfLambda>] ps : 'T PushStream) : _ array =
    let ra = toResizeArray 16 ps
    ra.ToArray ()

  let inline (|>>) ([<InlineIfLambda>] a : _ -> _) ([<InlineIfLambda>] f : _ -> _) = f a

module FsGen =
  module Details =
    open System
    open System.Reflection
    open System.Text

    open FSharp.Reflection

    type Gens = obj PushStream array
    type Ctor = obj array -> obj

    // All supported types have limited cardinality
    //  Gens are used to generate nested values for tuple types
    //  Ctor is used to create a tuple type
    //  A record is a tuple with a bit of sugar on top
    type SupportedType =
      | FsUnion   of (Gens*Ctor) array
      | FsRecord  of Gens*Ctor
      | FsTuple   of Gens*Ctor
      | FsUnit
      | Bool

    // Creates recursive receiver that populate vs
    //  At each step receiver the v from previous receiver and store in vs
    //  At the end of the receiver chain invoke the input receiver 
    let rec mkReceiverLoop (gs : Gens) (ctor : Ctor) (r : obj -> bool) (vs : obj array) i =
      if i < gs.Length - 1 then
        let next  = mkReceiverLoop gs ctor r vs (i + 1)
        let g     = gs.[i + 1]
        fun v ->
          vs.[i] <- v
          g next
      else
        fun v ->
          vs.[i] <- v
          r (ctor vs)

    let produceAll (gs : Gens) (ctor : Ctor) (r : obj -> bool) =
      if gs.Length > 0 then
        let vs = Array.zeroCreate gs.Length
        let g  = gs.[0]
        g (mkReceiverLoop gs ctor r vs 0)
      else
        r (ctor Array.empty)

    let rec genAll (p : string list) (t : Type) (remDepth : int) : obj PushStream =
      // In order to support recurisive types we need to limit to certain depth
      if remDepth <= 0 then
        fun r -> true
      else
        // Precompute the supported type so we don't have to do it for each value
        //  The ctors can be cached to reduce memory footprint
        let st = 
#if DEBUG
          printfn "Analyzing type: %s" t.FullName
#endif
          if t = typeof<bool> then
            Bool
          elif t = typeof<unit> then
            FsUnit
          elif FSharpType.IsUnion t then
            // Loop for all unit cases and create genarators and ctor for each case
            let mapper (case : UnionCaseInfo) =
              let ps = case.GetFields ()
              let gs =
                ps 
                |> Array.map (fun pi -> genAll ((sprintf "%s.%s" case.Name pi.Name)::p) pi.PropertyType (remDepth - 1))
              (gs, FSharpValue.PreComputeUnionConstructor case)
            FSharpType.GetUnionCases t
              |> Array.map mapper
              |> FsUnion
          elif FSharpType.IsRecord t then
            // Create generators for each nested type and ctor for record
            let ps = FSharpType.GetRecordFields t
            let gs =
              ps 
              |> Array.map (fun pi -> genAll ((sprintf "%s" pi.Name)::p) pi.PropertyType (remDepth - 1))
            (gs, FSharpValue.PreComputeRecordConstructor t) |> FsRecord
          elif FSharpType.IsTuple t then
            // Create generators for each nested type and ctor for tuple
            let ts = FSharpType.GetTupleElements t
            let gs = 
              ts 
              |> Array.mapi (fun ii ti -> genAll ((sprintf "_%i" ii)::p) ti (remDepth - 1))
            (gs, FSharpValue.PreComputeTupleConstructor t) |> FsTuple
          else
            // If the type isn't supported try to tell the programmer the
            //  path of the type in the type tree
            let sb = StringBuilder 16
            let rec loop (sb : StringBuilder) (pp : string list) =
              match pp with
              | []    -> sb.Append "$" |> ignore
              | h::t  -> 
                loop sb t
                sb.Append '.' |> ignore
                sb.Append h   |> ignore
            loop sb p
            failwithf "Unsupported type %s @ %s" t.Name <| sb.ToString ()
        fun r ->
          // r is a receiver of values so start producing values
          match st with
          | FsUnion cases ->
            // Loop from each union case and generator all values
            //  for each
            let rec cloop (cases : (Gens*Ctor) array) ci =
              if ci < cases.Length then
                let gs, ctor  = cases.[ci]
                produceAll gs ctor r && cloop cases (ci + 1)
              else
                true
            cloop cases 0
          | FsRecord (gs, ctor) ->
            produceAll gs ctor r
          | FsTuple (gs, ctor) ->
            let vs = Array.zeroCreate gs.Length
            produceAll gs ctor r
          | FsUnit ->
            // Produce a unit value
            r ()
          | Bool ->
            // Produce true and false
            r false && r true

  open PushStream
  // Produces a PushStream that will generate all values for 'T
  //  Cache this PushStream in order to avoid iterating over the type
  //  multiple times
  let inline genAll<'T> (maxDepth : int) : 'T PushStream =
    Details.genAll List.empty typeof<'T> maxDepth
      |>> map (fun (v : obj) -> v :?> 'T)

module TestJson =
  // A test model for a Json parser perhaps?
  type ArrayValue<'T> =
  | Empty
  | One of 'T
  | Two of 'T*'T

  type NumberValue =
  | Zero
  | One
  | PI

  type StringValue =
  | Empty
  | Hello
  | Escaped

  type JsonValue =
    | NullValue
    | StringValue   of StringValue
    | NumberValue   of NumberValue
    | BooleanValue  of bool
    | ArrayValue    of JsonValue ArrayValue
    | ObjectValue   of (StringValue*JsonValue) ArrayValue

  open PushStream
  let jsonValueGen  = FsGen.genAll<JsonValue> 6
  let genAll ()     = jsonValueGen |>> toArray
open System.Diagnostics

[<EntryPoint>]
let main argv =
  let sw = Stopwatch.StartNew ()
  let vs = TestJson.genAll ()
  sw.Stop ()
  printfn "%A" vs.Length
  printfn "Distinct: %A" (vs |> Array.distinct |> Array.length)
  printfn "Took %d ms" sw.ElapsedMilliseconds
  printfn "%A" (vs |> Array.truncate 10)
  0