{-# LANGUAGE GADTs, EmptyDataDecls #-}
module Expr where

type Identifier = String

data ContextRecord = VariableRecord Identifier Double
                   | FunctionRecord Identifier (ExprAST Function)

type EvalContext = [ContextRecord]

data BinaryOp a where
    AddOp       :: BinaryOp Double
    SubtractOp  :: BinaryOp Double
    MultiplyOp  :: BinaryOp Double
    DivideOp    :: BinaryOp Double

data Function

data ExprAST a where
    NumberExprAST   :: Double -> ExprAST Double
    VariableExprAST :: Identifier -> ExprAST a
    BinaryExprAST   :: BinaryOp Double -> ExprAST Double -> ExprAST Double -> ExprAST Double
    CallExprAST     :: Identifier -> ExprAST ()
    FunctionAST     :: Identifier -> [ExprAST a] -> ExprAST Function

{- Test eval -}
resolveOp :: BinaryOp a -> (a -> a -> a)
resolveOp AddOp = (+)
resolveOp SubtractOp = (-)
resolveOp MultiplyOp = (*)
resolveOp DivideOp = (/)

eval :: EvalContext -> ExprAST a -> a
eval ctx (NumberExprAST n) = n
eval ctx (BinaryExprAST op e1 e2) = (resolveOp op) (eval ctx e1) (eval ctx e2)