Haskell Cookbook

How to do it...

1. Create a new project working-with-rank-n-type with a simple stack template:

        stack new working-with-rank-n-type simple

2. Open src/Main.hs. We will be adding our source here.
3. Add the language extension Rank2Types, and define Main module:

        module Main where

4. Import the ST monad module:

 import Control.Monad.ST
 import Data.STRef
 import Control.Monad

5. Use the ST monad to calculate:

 factorialST :: (Num t, Eq t) => t -> STRef s t -> ST s t
 factorialST 0 x = readSTRef x
 factorialST n x = do
   x' <- readSTRef x
   writeSTRef x $! x' * n
   factorialST (n-1) x

6. Convert the preceding factorial function from ST s t to a pure function:

 factorial n = runST $ do
   x <- newSTRef 1
   factorialST n x

7. Call the factorial in main:

 main :: ...