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 this post I’ll be generating lists of random length.

The maximum length of the list will depend on the runtime size parameter. See this previous post that explains how size works.

Two generators are required for this:

- a generator that picks numbers with absolute value bounded by the size (that’s Gen’s
`choose`

) - a generator that creates lists of a
*given*length (that’s Gen’s`vector`

). The length will be the number obtained by the above generator

Gen’s `choose`

is already available. Porting Gen’s `vector`

, which essentially means porting replicateM

```
/// <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' =
Gen.bind m (fun x ->
Gen.bind m' (fun xs ->
Gen.init (x :: xs)))
Gen.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 =
Gen.sequence [ for _ in [ 1..n ] -> g ]
```

**Creating a custom gen computation expression**

Most, if not all, monadic computations on the `Gen<'a>`

type can be written in terms of `Gen.bind`

and `Gen.init`

(return).

A computation expression just makes this a whole lot easier:

```
[<AutoOpen>]
module Builder =
type GenBuilder() =
member this.Bind (g1, g2) = Gen.bind g1 g2
member this.ReturnFrom (f) = f
let gen = GenBuilder()
```

Given the above, a generator for lists of random length can be written as:

```
/// <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 = Gen.sized (fun s -> gen { let! n = Gen.choose (0, s)
return! Gen.vector n g })
```

Finally, here are some sample lists of type `int`

and `char`

, but it can be *any* generator of type `'a`

really

```
> Gen.int |> Gen.list |> Gen.generate;;
val it : int list = [-10; 1; 0; -14; 11; -2; -7]
> Gen.int |> Gen.resize 1337 |> Gen.list |> Gen.generate;;
val it : int list =
[-1027; 739; -571; 475; 979; 144; -1239; -1282; -1012; -932; -952]
> Gen.char |> Gen.list |> Gen.generate
val it : char list = [':'; 'e'; '«']
> Gen.char |> Gen.list |> Gen.generate
val it : char list =
['H'; 'µ'; 'Ø'; '°'; 'ò'; 'k'; '<'; 't'; 't'; 'È'; '\\'; '\151'; 'Æ'; '¥';
'/'; '\137'; 'é'; '\145'; '/'; 'Ì'; 'ï'; '\153'; '·'; '7']
```