On Fri, Jun 29, 2007 at 07:18:00PM +0100, Andrew Coppin wrote:
Dan Mead wrote:
Andrew: Try using catchalls in your guards
pattern1 | guard1 = | guard2 = | otherwise =
This makes it much easier to use pattern guards. "otherwise" is a reserved word used for this stuff in ghc.
Yeah, it's good practice to include an explicit otherwise clause - but in this specific instance, I'm trying to do tricky stuff where one rule falls through to another if some condition doesn't hold - but that condition is only expressible as a function.
Well, let me show you the code - somebody will probably recognise this stuff...
convert1 :: Expression -> Expression convert1 S = S convert1 K = K convert1 I = I convert1 (Var v) = Var v convert1 (x :@: y) = (convert1 x) :@: (convert1 y) convert1 (Lam n e) | n `not_in` e = K :@: (convert1 e) convert1 (Lam n (Var v)) | n == v = I | otherwise = K :@: (convert1 (Var v)) convert1 (Lam n (x :@: y)) | y `is_var` n && n `not_in` x = convert1 x | otherwise = S :@: (convert1 (Lam n x)) :@: (convert1 (Lam n y)) convert1 (Lam n (Lam m e)) | n `not_in` e = K :@: (convert1 (Lam m e)) | otherwise = convert1 (Lam n (convert1 (Lam m e)))
This is *much* easier expressed as a bottom-up traversal. compile = transform optimize . transform eliminate eliminate (Lam v e) = transform (abstract v) e eliminate x = x abstract v (Var v') | v == v' = I abstract v (a :@ b) = S :@ a :@ b abstract v x = x optimize (S :@ (K :@ x) :@ (K :@ y)) = K :@ (x :@ y) optimize (S :@ (K :@ x) :@ I) = x optimize x = x (Here using Uniplate, mostly because it is the freshest in my mind of all of them). Stefan