# Write you some QuickCheck - Generating random lists

19 February 2016

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

**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:

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

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

and `char`

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

really