Stupid Haskell Trick: Factorials in the Type System

[mvanier]

Ahh, summer! A time for backyard barbeques, ice cold beer, baseball games, and playing with experimental Haskell type system extensions. Or in my case, just the latter. In this article I'll show you how to compute factorials using Haskell's type system, i.e. at compile time.

Why would we want to do this? you may well ask. The real reason is

Comments

[wizzard0]

The most disturbing thing there is that i do understand how it works >_

once upon a time

[anonymous]

Just for a laugh, here's an antique type-level factorial function I cooked up a while back.

http://www.mail-archive.com/haskell-cafe@haskell.org/msg00692.html

I used the factorial to give a precise type to permutations, which I hope isn't entirely useless.

All the best

Conor

Re: once upon a time

[mvanier]

Conor,

I guess nothing in Haskell is ever completely useless ;-) Thanks for the link, I'll check it out!

More

[anonymous]

Now you need to extend it so you can use the type level factorial back at the value level again.

Re: More

[mvanier]

Yeah, I'll have to think about that.