∀QuickCheck-based clone — http://blog.nikosbaxevanis.com/2016/02/08/write-you-some-quickcheck/

LightCheck.fs

// Port of Haskell
// - https://hackage.haskell.org/package/QuickCheck-1.2.0.1
// - https://hackage.haskell.org/package/random-1.1

namespace LightCheck

/// <summary>
/// This module deals with the common task of pseudo-random number generation.
/// It makes it possible to generate repeatable results, by starting with a
/// specified initial random number generator, or to get different results on
/// each run by using the system-initialised generator or by supplying a seed
/// from some other source.
/// </summary>
/// <remarks>
/// This implementation uses the Portable Combined Generator of L'Ecuyer for
/// 32-bit computers, transliterated by Lennart Augustsson. It has a period of
/// roughly 2.30584e18.
/// </remarks>
[<AutoOpen>]
module internal Random =

    type StdGen =
        private
        | StdGen of int * int

    /// <summary>
    /// The next operation returns an Int that is uniformly distributed in the
    /// rangge of at least 30 bits, and a new generator. The result of repeatedly
    /// using next should be at least as statistically robust as the Minimal
    /// Standard Random Number Generator. Until more is known about implementations
    /// of split, all we require is that split deliver generators that are (a) not
    /// identical and (b) independently robust in the sense just given.
    /// </summary>
    let private next (StdGen (s1, s2)) =
        let k    = s1 / 53668
        let k'   = s2 / 52774
        let s1'  = 40014 * (s1 - k * 53668)  - k * 12211
        let s2'  = 40692 * (s2 - k' * 52774) - k' * 3791
        let s1'' = if s1' < 0 then s1' + 2147483563 else s1'
        let s2'' = if s2' < 0 then s2' + 2147483399 else s2'
        let z    = s1'' - s2''
        let z'   = if z < 1 then z + 2147483562 else z

        (z', StdGen (s1'', s2''))

    /// <summary>
    /// The split operation allows one to obtain two distinct random number
    /// generators. This is very useful in functional programs (for example, when
    /// passing a random number generator down to recursive calls), but very little
    /// work has been done on statistically robust implementations of split.
    /// </summary>
    let split (StdGen (s1, s2) as std) =
        let s1' = if s1 = 2147483562 then 1 else s1 + 1
        let s2' = if s2 = 1 then 2147483398 else s2 - 1
        let (StdGen (t1, t2)) = next std |> snd

        (StdGen (s1', t2), StdGen (t1, s2'))

    /// <summary>
    /// The range operation takes a range (lo,hi) and a random number generator g,
    /// and returns a random value, uniformly distributed, in the closed interval
    /// [lo,hi], together with a new generator.
    /// </summary>
    /// <remarks>
    /// It is unspecified what happens if lo > hi. For continuous types there is no
    /// requirement that the values lo and hi are ever produced, although they very
    /// well may be, depending on the implementation and the interval.
    /// </remarks>
    let rec range (l, h) rng =
        if l > h then range (h, l) rng
        else
            let (l', h')    = (32767, 2147483647)
            let b           = h' - l' + 1
            let q           = 1000
            let k           = h - l + 1
            let magnitude   = k * q
            let rec f c v g =
                if c >= magnitude then (v, g)
                else
                    let (x, g') = next g
                    let v'      = (v * b + (x - l'))
                    f (c * b) v' g'
            let (v, rng') = f 1 0 rng

            (l + v % k), rng'

    let private r = int System.DateTime.UtcNow.Ticks |> System.Random

    /// <summary>
    /// Provides a way of producing an initial generator using a random seed.
    /// </summary>
    let createNew() =
        let s       = r.Next() &&& 2147483647
        let (q, s1) = (s / 2147483562, s % 2147483562)
        let s2      = q % 2147483398

        StdGen (s1 + 1, s2 + 1)

/// <summary>
/// LightCheck exports some basic generators, and some combinators for making
/// new ones. Gen of 'a is the type for generators of 'a's and essentially is
/// a State Monad combining a pseudo-random generation seed, and a size value
/// for data structures (i.e. list length).
/// Using the type Gen of 'a, we can specify at the same time a set of values
/// that can be generated and a probability distribution on that set.
///
/// Read more about how it works here:
/// http://www.dcc.fc.up.pt/~pbv/aulas/tapf/slides/quickcheck.html#the-gen-monad
/// http://quviq.com/documentation/eqc/index.html
/// </summary>
module Gen =

    /// <summary>
    /// A generator for values of type 'a.
    /// </summary>
    type Gen<'a> =
        private
        | Gen of (int -> StdGen -> 'a)

    /// <summary>
    /// Sequentially compose two actions, passing any value produced by the first
    /// as an argument to the second.
    /// </summary>
    /// <param name="f">
    /// The action that produces a value to be passed as argument to the generator.
    /// </param>
    let bind (Gen m) f =
        Gen(fun n r ->
            let (r1, r2) = r |> Random.split
            let (Gen m') = f (m n r1)
            m' n r2)

    /// <summary>
    /// Injects a value into a generator.
    /// </summary>
    /// <param name="a">The value to inject into a generator.</param>
    let init a = Gen(fun n r -> a)

    /// <summary>
    /// Returns a new generator obtained by applying a function to an existing
    /// generator.
    /// </summary>
    /// <param name="f">The function to apply to an existing generator.</param>
    /// <param name="m">The existing generator.</param>
    let map f m =
        bind m (fun m' ->
            init (f m'))

    /// <summary>
    /// Generates a random element in the given inclusive range, uniformly
    /// distributed in the closed interval [lo,hi].
    /// </summary>
    /// <param name="lo">The lower bound.</param>
    /// <param name="hi">The upper bound.</param>
    let choose (lo, hi) = Gen(fun n r -> r) |> map (Random.range (lo, hi) >> fst)

    /// <summary>
    /// Generates one of the given values.
    /// </summary>
    /// <param name="xs">The input list.</param>
    /// <remarks>
    /// The input list must be non-empty.
    /// </remarks>
    let elements xs =
        // http://stackoverflow.com/a/1817654/467754
        let flip f x y = f y x
        choose (0, (Seq.length xs) - 1) |> map (flip Seq.item xs)

    /// <summary>
    /// Randomly uses one of the given generators.
    /// </summary>
    /// <param name="gens">The input list of generators to use.</param>
    /// <remarks>
    /// The input list must be non-empty.
    /// </remarks>
    let oneof gens =
        let join x = bind x id
        join (elements gens)

    /// <summary>
    /// Used to construct generators that depend on the size parameter.
    /// </summary>
    /// <param name="g">A generator for values of type 'a.</param>
    let sized g =
        Gen(fun n r ->
            let (Gen m) = g n
            m n r)

    /// <summary>
    /// Overrides the size parameter. Returns a generator which uses the given size
    /// instead of the runtime-size parameter.
    /// </summary>
    /// <param name="n">The size that's going to override the runtime-size.</param>
    let resize n (Gen m) = Gen(fun _ r -> m n r)

    /// <summary>
    /// Takes a list of generators of type 'a, evaluates each one of them, and
    /// collect the result, into a new generator of type 'a list.
    /// </summary>
    /// <param name="l">The list of generators of type 'a.</param>
    /// <remarks>
    /// This is written so that the F# compiler will use a tail call, as shown in
    /// the resulting excerpt of generated IL:
    ///   IL_0000: nop
    ///   IL_0001: call class [FSharp.Core]Microsoft.FSharp.Core.FSharpFunc`2<cl...
    ///   IL_0006: ldarg.0
    ///   IL_0007: call class [FSharp.Core]Microsoft.FSharp.Collections.FSharpLi...
    ///   IL_000c: call class LightCheck.Gen/Gen`1<!!0> LightCheck.Gen::'init'<c...
    ///   IL_0011: tail.
    ///   IL_0013: call !!1 [FSharp.Core]Microsoft.FSharp.Collections.ListModule...
    ///   IL_0018: ret
    /// See also:
    ///   http://stackoverflow.com/a/6615060/467754,
    ///   http://stackoverflow.com/a/35132220/467754
    /// </remarks>
    let sequence l =
        let k m m' =
            bind m (fun x ->
                bind m' (fun xs ->
                    init (x :: xs)))
        init [] |> List.foldBack k l

    /// <summary>
    /// Generates a list of the given length.
    /// </summary>
    /// <param name="n">The number of elements to replicate.</param>
    /// <param name="g">The generator to replicate.</param>
    let vector n g =
        sequence [ for _ in [ 1..n ] -> g ]

    [<AutoOpen>]
    module Builder =
        type GenBuilder() =
            member this.Bind       (g1, g2) = bind g1 g2
            member this.Return     (x)      = init x
            member this.ReturnFrom (f)      = f

        let gen = GenBuilder()

    /// <summary>
    /// Generates a list of random length. The maximum length of the list depends
    /// on the size parameter.
    /// </summary>
    /// <param name="g">The generator from which to create a list from.</param>
    let list g = sized (fun s -> gen { let! n = choose (0, s)
                                       return! vector n g })

    /// <summary>
    /// Unpacks a function wrapped inside a generator, applying it into a new
    /// generator.
    /// </summary>
    /// <param name="f">The function wrapped inside a generator.</param>
    /// <param name="m">The generator, to apply the function to.</param>
    let apply f m =
        bind f (fun f' ->
            bind m (fun m' ->
                init (f' m')))

    /// <summary>
    /// Returns a new generator obtained by applying a function to three existing
    /// generators.
    /// </summary>
    /// <param name="f">The function to apply to the existing generators.</param>
    /// <param name="m1">The existing generator.</param>
    /// <param name="m2">The existing generator.</param>
    /// <param name="m3">The existing generator.</param>
    let lift3 f m1 m2 m3 = apply (apply (apply (init f) m1) m2) m3

    /// <summary>
    /// Generates a random byte.
    /// </summary>
    let byte = choose (0, 255) |> map Operators.byte
 
    /// <summary>
    /// Generates a random character.
    /// </summary>   
    let char =
        oneof [ choose ( 32, 126)
                choose (127, 255) ]
        |> map Operators.char

    /// <summary>
    /// Generates a random boolean.
    /// </summary>
    let bool =
        oneof [ init true
                init false ]
    
    /// <summary>
    /// Generates a 32-bit integer (with absolute value bounded by the generation
    /// size).
    /// </summary>
    let int = sized (fun n -> choose (-n, n))

    /// <summary>
    /// Generates a 64-bit integer (with absolute value bounded by the generation
    /// size multiplied by 16-bit integer's largest possible value).
    /// </summary>    
    let int64 = int |> map (fun n -> Operators.int64 (n * 32767))

    /// <summary>
    /// Generates a random string.
    /// </summary>
    let string =
        char
        |> list
        |> map (List.toArray >> System.String)

    /// <summary>
    /// Generates a random real number.
    /// </summary>
    let float =
        let fraction a b c = float a + float b / (abs (float c) + 1.0)
        lift3 fraction int int int

    /// <summary>
    /// Runs a generator. The size passed to the generator is up to 30; if you want
    /// another size then you should explicitly use 'resize'.
    /// </summary>
    let generate (Gen m) =
        let (size, rand) = Random.createNew() |> Random.range (0, 30)
        m size rand

    /// <summary>
    /// Generates some example values.
    /// </summary>
    /// <param name="g">The generator to run for generating example values.</param>
    let sample g =
        [ for n in [ 0..2..20 ] -> resize n g |> generate ]

/// <summary>
/// This module deals with simplifying counter-examples. A property fails when
/// LightCheck finds a first counter-example. However, randomly-generated data
/// typically contains a lot of noise. Therefore it is a good idea to simplify
/// counter-examples before reporting them. This process is called shrinking.
///
/// Read more about how it works here:
/// http://www.dcc.fc.up.pt/~pbv/aulas/tapf/slides/quickcheck.html#shrinking
/// </summary>
module Shrink =
    
    open FSharp.Core.LanguagePrimitives
    
    /// <summary>
    /// A shrinker for values of type 'a.
    /// </summary>
    type Shrink<'a> =
        private
        | Shrink of ('a -> 'a seq)
    
    /// <summary>
    /// Shrinks towards smaller numeric values.
    /// </summary>
    /// <param name="n">The numeric value to shrink.</param>
    let inline shrinkNumber n =
        let genericTwo = GenericOne + GenericOne
        n
        |> Seq.unfold (fun s -> Some(n - s, s / genericTwo))
        |> Seq.tail
        |> Seq.append [ GenericZero ]
        |> Seq.takeWhile (fun el -> abs n > abs el)
        |> Seq.append (if n < GenericZero then Seq.singleton -n
                       else Seq.empty)
        |> Seq.distinct
    
    /// <summary>
    /// Shrinks a sequence of elements of type 'a. First it yields an empty
    /// sequence, and then it iterates the input sequence, and shrinks each
    /// one of the items given the shrinker which is passed as a parameter.
    /// </summary>
    /// <param name="f">
    /// The shrinker function, to be applied on each element of the list.
    /// </param>
    /// <param name="xs">The input sequence to shrink.</param>
    let shrinkList xs (Shrink shr) =
        let rec shrinkImp xs =
            match xs with
            | []       -> Seq.empty
            | (h :: t) ->
                seq {
                    yield []
                    for h' in        shr h  -> h' :: t
                    for t' in (shrinkImp t) -> h  :: t'
                }
        shrinkImp xs

module Property =

    open Gen

    /// <summary>
    /// A generator of values Gen<Result>, in order to make it possible to mix and
    /// match Property combinators and Gen computations.
    /// </summary>
    type Property =
        private
        | Prop of Gen<Result>

    and Result =
        { Status : option<bool>
          Stamps : list<string>
          Args   : list<string> }

    /// <summary>
    /// Returns a value of type Gen Result out of a property. Useful for mixing and
    /// matching Property combinators and Gen computations.
    /// </summary>
    /// <param name="property">A property to extract the Gen Result from.</param>
    let evaluate property =
        let (Prop result) = property
        result

    let private boolProperty a =
        { Status = Some a
          Stamps = []
          Args   = [] }
        |> Gen.init
        |> Prop

    let private unitProperty =
        { Status = None
          Stamps = []
          Args   = [] }
        |> Gen.init
        |> Prop

    let private convert candidate =
        match box candidate with
        | :? Lazy<bool> as b -> boolProperty b.Value
        | :? Property   as p -> p
        | :? bool       as b -> boolProperty b
        | _                  -> unitProperty

    /// <summary>
    /// Returns a property that holds for all values that can be generated by Gen.
    /// </summary>
    /// <param name="g">A generator of values for which the property holds.</param>
    /// <param name="f">
    /// The property for checking whether it holds for all values that can be
    /// generated by a given Gen.
    /// </param>
    let forAll g f =
        Prop(gen {
                 let! arg = g
                 let! res = f arg
                            |> convert
                            |> evaluate
                 return { res with Args = arg.ToString() :: res.Args }
             })

    /// <summary>
    /// Returns a property that holds under certain conditions. Laws which are
    /// simple equations are conveniently represented by boolean function, but in
    /// general many laws hold only under certain conditions.
    /// This implication combinator represents such conditional laws.
    /// </summary>
    /// <param name="b">The precondition's predicate result.</param>
    /// <param name="a">The actual result, to be turned into a property.</param>
    let implies b a =
        if b then a |> convert
        else     () |> convert

    /// <summary>
    /// Returns a property that holds under certain conditions. Laws which are
    /// simple equations are conveniently represented by boolean function, but in
    /// general many laws hold only under certain conditions.
    /// This implication combinator represents such conditional laws.
    /// </summary>
    /// <param name="b">The precondition's predicate result.</param>
    /// <param name="a">The actual result, to be turned into a property.</param>
    let (==>) b a = implies b a

    /// <summary>
    /// Labels a test case.
    /// </summary>
    /// <param name="s">The label.</param>
    /// <param name="a">The test case.</param>
    let label s a =
        a
        |> evaluate
        |> map (fun result -> { result with Stamps = s :: result.Stamps })
        |> Prop

    /// <summary>
    /// Conditionally labels a test case.
    /// </summary>
    /// <param name="b">
    /// The condition to check whether the test case should be labelled.
    /// </param>
    /// <param name="s">The label.</param>
    /// <param name="a">The test case.</param>
    let classify b s a =
        if b then a |> label s
        else     () |> convert

    /// <summary>
    /// Conditionally labels a test case as trivial.
    /// </summary>
    /// <param name="b">
    /// The condition to check whether the test case should be labelled as trivial.
    /// </param>
    /// <param name="s">The label.</param>
    /// <param name="a">The test case.</param>
    let trivial b p = classify b "trivial" p

    /// <summary>
    /// Gathers all values that are passed to it.
    /// </summary>
    /// <param name="a">The value.</param>
    /// <param name="p">The property.</param>
    let collect a p = label (a.ToString()) p