Write you some QuickCheck  Generating random bytes
This post is part of a series of posts on implementing a minimal version of QuickCheck from scratch. The source code is available on GitHub.
We already have a function that runs a generator Gen<'a>
and returns random test data of type 'a
.
val generate : Gen<'a> > 'a
Now, in order to be able to generate random bytes, we need a couple of generators:
 a generator that picks numbers from a given range (in this case [0..255])
 a generator that applies a function (in this case
Operators.byte<^T>
) to an existing generator
Porting Gen’s choose
/// <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) > Gen.map (Random.range (lo, hi) >> fst)
Porting Gen’s map
, bind
, and return
^{1} since choose
depends on them
/// <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'))
All the pieces are now in place, and so a byte generator can be written as:
let byte = Gen.choose (0, 255) > Gen.map Operators.byte
val byte : Gen<byte>
Finally, here are some sample bytes:
> Gen.byte > Gen.generate;;
val it : byte = 125uy
> Gen.byte > Gen.generate;;
val it : byte = 201uy
> Gen.byte > Gen.generate;;
val it : byte = 3uy

In F#,
return
is already taken, so I usedinit
instead. ↩