pars.hs

parseIf :: Parser Statement
parseIf = do
  spaces
  string "if "
  spaces
  expr <- expression
  spaces
  string "then "
  st <- statement
  return $ If expr st Nothing

parseIfElse :: Parser Statement
parseIfElse = do
  spaces
  string "if "
  spaces
  expr <- expression
  spaces
  string "then "
  st <- statement
  spaces
  string "else "
  mst <- statement
  return $ If expr st $ Just mst

statement :: Parser Statement
statement = try parseIfElse <|> parseIf <|> assignment