Simple esolang. Full source code for http://mroman.ch/impleso.html

eso.hs

import Text.ParserCombinators.Parsec
import Text.ParserCombinators.Parsec.Combinator
import Text.ParserCombinators.Parsec.Expr
import Text.ParserCombinators.Parsec.Token

import Data.Maybe

data Expression = 
   Loop [Expression]
 | Literal Integer
 | Instruction Char
 deriving (Show, Read)

runParserWithString p input =
  case parse p "" input of
    Left err -> error $ show err
    Right q -> q

parseInteger :: Parser Expression
parseInteger = do
  optional spaces
  sign <- optionMaybe $ char '-'
  digits <- many1 $ oneOf "0123456789"
  optional spaces
  case sign of
    Just q -> return $ Literal . read $ "-" ++ digits
    Nothing -> return $ Literal . read $ digits
    
parseInstruction :: Parser Expression
parseInstruction = do
  optional spaces
  ins <- oneOf "+-*/^v\\$"
  optional spaces
  return $ Instruction ins
  
parseLoop :: Parser Expression
parseLoop = do
  optional spaces
  char '('
  optional spaces
  exps <- parseExpressions
  optional spaces
  char ')'
  optional spaces
  return $ Loop exps
  
parseExpressions :: Parser [Expression]
parseExpressions = many $ (try parseInteger) <|> parseInstruction <|> parseLoop

eval xs = evalExps xs ([],[])

evalExps (x:xs) state = evalExps xs (evalExp x state)
evalExps [] state = state

evalExp (Literal i) (ms, ss) = (i : ms, ss)
evalExp (Instruction '+') ((b:a:ms),ss) = ((a+b) : ms, ss)
evalExp (Instruction '-') ((b:a:ms),ss) = ((a-b) : ms, ss)
evalExp (Instruction '*') ((b:a:ms),ss) = ((a*b) : ms, ss)
evalExp (Instruction '/') ((b:a:ms),ss) = ((a `div` b) : ms, ss)
evalExp (Instruction 'v') ((a:ms),ss) = (ms, a : ss)
evalExp (Instruction '$') (ms,(a:ss)) = (a : ms, ss)
evalExp (Instruction '^') ((a:ms),ss) = (a : a : ms, ss)
evalExp (Instruction '\\') ((a:b:ms),ss) = (b : a : ms, ss)

evalExp (Loop exps) (0 : ms, ss) = (0 : ms, ss)
evalExp l@(Loop exps) (a : ms, ss) = evalExp l (evalExps exps (a : ms, ss))

evalExp q s = error $ show q