Nanoid package is an implementation of ai/nanoid in Haskell.

Nanoid is a tiny, secure, URL-friendly, unique string ID generator

$ cabal new-repl
GHCi, version 8.2.2: http:https://www.haskell.org/ghc/  :? for help
[1 of 1] Compiling Nanoid           ( Nanoid.hs, interpreted )
Ok, one module loaded.
*Nanoid> generate [10..20] url 5
"89abc"

*Nanoid> generate <$> randomsIO <*> pure "abcd" <*> pure 4
"dabb"

*Nanoid> generate <$> randomsIO <*> pure url <*> pure 21
"1xfg7ez1WL7r6jinFzTsy"

The core nanoid generator function is the point of our interest. The goal of this project is not to implement a UID generator in Haskell, but to replicate the original nanoid's algorithm. We'll see how to express imperative language constructs like variables, mutation, and for-loops in Haskell and get additional guarantees about the stateful program. The ultimate idea is to show that functional language can be as practical as an imperative one, but with a much higher expressiveness that allows writing programs in different styles.

This package is written as an article in a documentation to the Nanoid.hs module, and serves purely educational purposes. I choosed ai/nanoid because it is a useful real-world program, it is small, it has mutability and side effects, and it already has been ported to other programming lanugages.

The technique which I used is not new and was described in Lazy functional state threads paper.

You can also check Imperative Hakskell blog post on this topic. It uses the same approach to convert classic CS algorightms to Haskell.

All the examples are runnable from GHCi REPL provided by the awesome Cabal new build infrastructure.

$ cabal new-repl
In order, the following will be built (use -v for more details):
 - nanoid-hs-0.1.0.0 (lib) (file Nanoid.hs changed)
Preprocessing library for nanoid-hs-0.1.0.0..
GHCi, version 8.2.2: https://www.haskell.org/ghc/  :? for help
[1 of 1] Compiling Nanoid           ( Nanoid.hs, interpreted )
Ok, one module loaded.
*Nanoid>

Generate HTML version of the package documentation with:

$ cabal new-haddock

• nanoid https://github.com/ai/nanoid
• Lazy functional state threads https://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.144.2237
• Imperative Haskell https://vaibhavsagar.com/blog/2017/05/29/imperative-haskell/