Example Haskell implementation requested by SrPeixinho.

Main.hs

-- In response to [1].
-- 
-- I am aiming for readability, not conciseness. Nevertheless, a note
-- explaining why the original javascript code is so much shorter:
-- Javascript skips all error checking unless you perform explicit checks.
-- Haskell enforces all error checking unless you explicitly ignore errors.
-- I have thus implemented many wrapping functions whose purpose is to use
-- incomplete pattern-matching to ignore errors.
-- 
-- I am using the aeson package for JSON and the download-curl package for
-- network access. I haven't used either library before, so that part might
-- not be idiomatic.
-- 
-- [1] http://www.reddit.com/r/haskell/comments/1zy7ut/how_would_this_program_be_implemented_in_haskell/
{-# LANGUAGE OverloadedStrings #-}
module Main where

import qualified Control.Concurrent as C
import qualified Data.Aeson as Json
import qualified Data.ByteString as B
import qualified Data.Text as T
import qualified Data.HashMap.Strict as HashMap
import qualified Network.Curl.Download as Curl
import qualified System.Random as R

import Control.Applicative
import Control.Monad
import Text.Printf


-- sleep uses seconds, instead of threadDelay's unintuitive microseconds.
sleep :: Double -> IO ()
sleep seconds = C.threadDelay (floor (1000000 * seconds))


-- pick a random element from a list, crashing if the list is empty.
randomFrom :: [a] -> IO a
randomFrom xs = do
    i <- R.randomRIO (0, length xs - 1)
    return (xs !! i)


-- a version of Json.fromJSON which crashes on error.
fromJson :: Json.FromJSON a => Json.Value -> a
fromJson v = let Json.Success x = Json.fromJSON v in x

-- a version of Json.(.:) usable outside of FromJSON instances.
(.:) :: Json.FromJSON a => Json.Value -> T.Text -> a
Json.Object o .: key = let defaultValue = error "key not found"
                           value = HashMap.lookupDefault defaultValue key o
                        in fromJson value


type RestURL = String
type FullURL = String

pokemonUrl :: Int -> RestURL
pokemonUrl = printf "/api/v1/pokemon/%d"

wrapRestUrl :: RestURL -> FullURL
wrapRestUrl = printf "http://viclib.com/pokemon?query=%s"

-- download a URL, crashing on error.
downloadByteString :: FullURL -> IO B.ByteString
downloadByteString url = do
    Right contents <- Curl.openURI url
    return contents

-- fetch some JSON, crashing on error.
downloadJson :: FullURL -> IO Json.Value
downloadJson url = do
    Just json <- Json.decodeStrict <$> downloadByteString url
    return json


getRandomPokemon :: IO Json.Value
getRandomPokemon = do
    pokemon_number <- R.randomRIO pokemonRange
    let url = pokemonUrl pokemon_number
    downloadJson (wrapRestUrl url)
  where
    -- only use first-generation Pokémons.
    -- 
    -- BUGFIX: the javascript version sometimes requests Pokémon number 0,
    --         which doesn't exist.
    pokemonRange :: (Int, Int)
    pokemonRange = (1, 151)

getRandomMove :: [Json.Value] -> IO Json.Value
getRandomMove moveset = do
    move <- randomFrom moveset
    let url = move .: "resource_uri"
    downloadJson (wrapRestUrl url)

main :: IO ()
main = do
    -- Find random Pokémon
    pokemon <- getRandomPokemon
    let pokemon_name = pokemon .: "name" :: String
    printf "Wild %s appeared!\n" pokemon_name
    
    -- Download 4 moves from valid moveset
    let moveset = pokemon .: "moves" :: [Json.Value]
    moves <- replicateM 4 (getRandomMove moveset)
    
    -- Every 1.5 seconds it attacks you
    let loop hp move_count = unless (hp <= 0) $ do
        move <- randomFrom moves
        let move_name = move .: "name" :: String
        let max_dmg   = move .: "power"
                      * (pokemon .: "attack" + pokemon .: "sp_atk")
                  `div` 300
        dmg <- R.randomRIO (0, max_dmg)
        
        let hp'         = hp - dmg       :: Int
        let move_count' = move_count + 1 :: Int
        printf "%s attacked you with *%s* dealing %d damage!\n"
               pokemon_name move_name dmg
        if hp' > 0
          then printf "You have %d hp!\n" hp'
          else printf "%s defeated you in %d moves.\n" pokemon_name move_count
        
        sleep 1.5
        loop hp' move_count'
    loop 200 0