Testing Code

This post is just a test of source code formatting. OK, So source code formatting works fine inside github but not inside the actual blog.. not sure how to sort that.

Here is a useful thing called the “Fresh” monad:

```haskell {-# LANGUAGE GeneralizedNewtypeDeriving #-}

module Fresh ( Fresh(..), runFresh, fresh ) where

import Control.Monad.State

newtype Fresh s a = Fresh { unFresh :: State [s] a } deriving Monad

runFresh m s = evalState (unFresh m) s

fresh = do (x:xs) <- Fresh get ; Fresh (put xs) ; return x

```

you give it an infinite stream of distinct values and it lets you get out a fresh one every single time.

Just for fun here is the same sort of thing in the Eff language:

``` type fresh = effect operation gensym : unit -> int end

let run s = handler | val y -> (fun s -> y) | s#gensym () k -> (fun s -> k s (s+1)) | finally f -> f 0;;

let g = new fresh in with run g handle g#gensym () :: g#gensym () :: g#gensym () :: [] ;; ```