pthariensflame

The Indexed State Monad in Haskell, Scala, and C#

Have you ever had to write code that made a complex series of succesive modifications to a single piece of mutable state? (Almost certainly yes.)

Did you ever wish you could make the compiler tell you if a particular operation on the state was illegal at a given point in the modifications? (If you're a fan of static typing, probably yes.)

If that's the case, the indexed state monad can help!

Motivation

tonymorris

using System;
using System.Collections;
using System.Collections.Generic;

/*
A basic lens library for the purpose of demonstration.
Implements a lens as the costate comonad coalgebra.

This library is not complete.
A more complete lens library would take from

jboner

Latency Comparison Numbers (~2012)
----------------------------------
L1 cache reference                           0.5 ns
Branch mispredict                            5   ns
L2 cache reference                           7   ns                      14x L1 cache
Mutex lock/unlock                           25   ns
Main memory reference                      100   ns                      20x L2 cache, 200x L1 cache
Compress 1K bytes with Zippy             3,000   ns        3 us
Send 1K bytes over 1 Gbps network       10,000   ns       10 us
Read 4K randomly from SSD*             150,000   ns      150 us          ~1GB/sec SSD

jvranish

{-# LANGUAGE Rank2Types
           , RebindableSyntax
           , ImplicitParams
           , NoMonomorphismRestriction #-}

import Data.Maybe
import Data.Function
import Data.String
import Prelude (undefined, error, String, (++))

forki

// Mimic type classes with additional param

type 'a Num = {
    zero: 'a
    add: 'a -> 'a -> 'a
    mult: 'a -> 'a -> 'a
    fromInteger: int -> 'a }

let intNum = {
    zero = 0