-- filename: evalenv.ads 2:47PM  10/25/95

-- evaluation environments are stacks of symbol tables (frames)


with framelist.stacks, expr.io, Frame;
use framelist.stacks, expr.io, expr, framelist;

package EvalEnv is

  type Env is new Stack;

-- in order to debug things we need to look at environments
  procedure put(E:in Env);

  procedure Initialize(E:in out Env);

  procedure AddFrame(F: in Frame.Table; E: in out Env) renames push;

  procedure FlushFrame(E: in out Env) renames Pop;
  
  procedure Assign(OldEnv:in Env; ToEnv: in out Env) renames MkSame;

  function FirstFrame(E: in Env) return Frame.Table renames TopOf;

--  function Is_Empty is already there

  function RestOf(E:Env) return Env;

-- the following procedures and functions operate on environments
-- by operating on the frames in the environments, thus many are 
-- defined in terms of similar operations on frames

  procedure AddEntry(S: in Symbol; V: in Exp; E: in out Env);

  procedure ChangeEntry(S:in Symbol; V: in Exp; E:in out Env);

  function Lookup(S:in Symbol; E:in Env) return Exp;

  function IsBound(S: in Symbol; E:in Env) return Boolean;

  procedure ExtendEnv(Names:in SymbolList; Vals:in ExpList;
                      E:in out Env);

end EvalEnv;