Write you some QuickCheck - Generating random characters

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.

In the previous post I generated random bytes. In this post, I’ll be generating random characters.

I’m going to need a couple of generators for this:

one that picks numbers from the range (in HEX)¹
- [20h..7Eh] for ASCII printable characters
- [80h..FFh] for ASCII extended characters
a generator that randomly uses one of the above generators
a generator that applies a function (in this case Operators.char<^T>) to the above

Porting Gen’s oneof:

/// <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 = Gen.bind x id
    join (Gen.elements gens)

Porting Gen’s elements since oneof depends on it:

/// <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
    Gen.choose (0, (Seq.length xs) - 1) |> Gen.map (flip Seq.item xs)

All the pieces are now in place, and so a char generator can be written as:

let char =
    Gen.oneof [ Gen.choose ( 32, 126)
                Gen.choose (127, 255) ]
    |> Gen.map Operators.char

val char : Gen<char>

Finally, here are some sample characters:

> Gen.char |> Gen.generate;;
val it : char = 'æ'

> Gen.char |> Gen.generate;;
val it : char = 'W'

> Gen.char |> Gen.generate;;
val it : char = 'h'

> Gen.char |> Gen.generate;;
val it : char = 'à'

> Gen.char |> Gen.generate;;
val it : char = 'ì'

> Gen.char |> Gen.generate;;
val it : char = '&'

> Gen.char |> Gen.generate;;
val it : char = ' '

> Gen.char |> Gen.generate;;
val it : char = 'o'

> Gen.char |> Gen.generate;;
val it : char = 'X'

I used the ASCII codes from this table. ↩