{
 The Lambda Shell, an interactive environment for evaluating pure untyped lambda terms.
 Copyright (C) 20052011, Robert Dockins

 This program is free software; you can redistribute it and/or modify
 it under the terms of the GNU General Public License as published by
 the Free Software Foundation; either version 2 of the License, or
 (at your option) any later version.

 This program is distributed in the hope that it will be useful,
 but WITHOUT ANY WARRANTY; without even the implied warranty of
 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
 GNU General Public License for more details.

 You should have received a copy of the GNU General Public License
 along with this program; if not, write to the Free Software
 Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 021101301 USA
}
module Env
( Env
, empty
, insert
, lookup
) where
import Prelude hiding (lookup)
import qualified Data.Set as Set
data Env = Env !Int ![String] !(Set.Set String)
empty :: Set.Set String > Env
empty = Env 0 []
insert :: String > Env > Env
insert label (Env z labels set)
 label `Set.member` set = Env (z+1) ( (label++"_"++(show z)) : labels) set
 otherwise = Env z ( label : labels ) (Set.insert label set)
lookup :: Int > Env > String
lookup x (Env z labels set) =
case drop x labels of
l:_ > l
[] > error (concat ["'",show x,"' out of bounds in environment ",show labels])