-- Sample implementation of classic OO patterns in FLP.
-- It includes: Composite, Interpreter and Visitor.
-- Sergio Antoy
-- Tue Jul 17 08:32:17 PDT 2001
-- Updated Mon Apr 18 09:03:37 PDT 2011

import Store

-- Composite-like representation of the abstract syntax of simple
-- expressions made up by literal integers, variables, and arithmetic
-- and boolean operators.  Boolean operations return 1 for true and
-- 0 for false.

data Expr
  = Num Int
  | Var String
  -- | Neg Expr
  | Add Expr Expr
  | Sub Expr Expr
  | Mul Expr Expr
  -- | Div Expr Expr
  | Gt Expr Expr

-- Evaluate an expression in a give environment.
-- The environment is the second argument of the function.
-- Expressions are evaluated without side effects on the environment.

evaluate (Num num) _ = num
evaluate (Var var) z | get var num z = num where num free
evaluate (Add e1 e2) z = (evaluate e1 z) + (evaluate e2 z)
evaluate (Sub e1 e2) z = (evaluate e1 z) - (evaluate e2 z)
evaluate (Mul e1 e2) z = (evaluate e1 z) * (evaluate e2 z)
evaluate (Gt e1 e2) z = if (evaluate e1 z > evaluate e2 z) then 1 else 0

-- A couple of simple tests of expression construction and evaluation.

Expr_test1 = evaluate (Sub (Num 8) (Num 5)) init_store
Expr_test3 = print (evaluate (Var "var") z)
  where z = set "var" 4 init_store

-- end of module