PushNegFExt.hs

{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE NoMonomorphismRestriction #-}

-- * Demonstrating `non-compositional', context-sensitive processing
-- * Extending the final style

module PushNegFExt where

-- Explain the imports
import Intro2 hiding (main)		-- Exp in the final form
import PushNegF hiding (main)		-- Push_neg interpreter
import ExtF hiding (main)		-- `mul' extension

-- * //
-- But the multiplication is not a homomorphism with respect to negation!
-- * neg (a * b) /= (neg a) * (neg b)
instance MulSYM repr => MulSYM (Ctx -> repr) where
    mul e1 e2 Pos = mul (e1 Pos) (e2 Pos)
    mul e1 e2 Neg = mul (e1 Pos) (e2 Neg)

-- Let us recall how an extended term looked like
tfm1_view = view tfm1
-- "(7 + (-(1 * 2)))"

tfm1_eval = eval tfm1
-- 5


tfm1_norm = push_neg tfm1

-- The new expression can be evaluated with any interpreter
tfm1_norm_view = view tfm1_norm
-- "(7 + ((-1) * 2))"
-- The result of the standard evaluation (the `meaning') is preserved
tfm1_norm_eval = eval tfm1_norm
-- 5

-- Add an extra negation
tfm1n_norm = push_neg (neg tfm1)

-- see the result
tfm1n_norm_view = view tfm1n_norm
-- "((-7) + (1 * 2))"
tfm1n_norm_eval = eval tfm1n_norm
-- -5

-- Negate the already negated term
tfm1nn_norm = push_neg (neg tfm1n_norm)
tfm1nn_norm_view = view tfm1nn_norm
-- "(7 + ((-1) * 2))"
tfm1nn_norm_eval = eval tfm1nn_norm
-- 5

-- The same for tmf2
-- We can even use a previously defined unextended expression (tf1)
-- as a part of the extended expression.
-- We can indeed mix-and-match

tfm2_view = view tfm2
-- "(7 * (8 + (-(1 + 2))))"

tfm2_eval = eval tfm2
-- 35


tfm2_norm = push_neg tfm2

tfm2_norm_view = view tfm2_norm
-- "(7 * (8 + ((-1) + (-2))))"
tfm2_norm_eval = eval tfm2_norm
-- 35

-- Add an extra negation
tfm2n_norm = push_neg (neg tfm2)

-- see the result
tfm2n_norm_view = view tfm2n_norm
-- "(7 * ((-8) + (1 + 2)))"
tfm2n_norm_eval = eval tfm2n_norm
-- -35

-- Negate the already negated term
tfm2nn_norm = push_neg (neg tfm2n_norm)
tfm2nn_norm_view = view tfm2nn_norm
-- "(7 * (8 + ((-1) + (-2))))"
tfm2nn_norm_eval = eval tfm2nn_norm
-- 35


main = do
       print PushNegF.tf1_norm_view -- old terms still work
       print PushNegFExt.tfm1_view
       print PushNegFExt.tfm1_eval
       print tfm1_norm_view
       print tfm1_norm_eval
       print tfm1n_norm_view
       print tfm1n_norm_eval
       print tfm1nn_norm_view
       print tfm1nn_norm_eval
       if tfm1_norm_view == tfm1nn_norm_view then return ()
	  else error "Double neg"
       if PushNegFExt.tfm1_eval == tfm1_norm_eval then return ()
	  else error "Normalization"
       if PushNegFExt.tfm1_eval == - tfm1n_norm_eval then return ()
	  else error "Normalization"

       print PushNegFExt.tfm2_view
       print PushNegFExt.tfm2_eval
       print tfm2_norm_view
       print tfm2_norm_eval
       print tfm2n_norm_view
       print tfm2n_norm_eval
       print tfm2nn_norm_view
       print tfm2nn_norm_eval
       if tfm2_norm_view == tfm2nn_norm_view then return ()
	  else error "Double neg"
       if PushNegFExt.tfm2_eval == tfm2_norm_eval then return ()
	  else error "Normalization"
       if PushNegFExt.tfm2_eval == - tfm2n_norm_eval then return ()
	  else error "Normalization"