Hi, Below is a function that returns a mirror of a tree, originally from: http://www.nijoruj.org/~as/2009/04/20/A-little-fun.html
where it was used to demonstrate the use of Haskabelle(1) which converts Haskell programs to the Isabelle theorem prover. Isabelle was used to show that the Haskell implementation of mirror is a model for the axiom: mirror (mirror x) = x My question is this: Is there any way to achieve such a proof in Haskell itself? GHC appears to reject equations such has mirror (mirror x) = x mirror (mirror(Node x y z)) = Node x y z Regards, Pat =================CODE===================== module BTree where data Tree a = Tip | Node (Tree a) a (Tree a) mirror :: Tree a -> Tree a mirror (Node x y z) = Node (mirror z) y (mirror x) mirror Tip = Tip (1)Thanks to John Ramsdell for the link to Haskabelle: http://www.cl.cam.ac.uk/research/hvg/Isabelle/haskabelle.html). _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe