2009/3/23 Claus Reinke
I just noticed that GHC (6.11.20090320) seems to compile both
f (a:b:c) = f (a:[]) = f [] = and f [] = f (a:[]) = f (a:b:c) =
to something like (looking at Core, but writing source)
f x = case x of { [] -> ..; (a:t) -> case t of { [] ->..; (b:c) ->..}}
In Core, case alternatives are stored in an order determined by the "data constructor tag" of the thing being matched on - this is independent of the order you wrote them in the source code. I believe the reason for this is just to reduce the work done by the code generator a tiny bit (and it's sometimes handy to know that the default case, if any, is always the first alternative). I don't know if preserving the order of the alts as written by the user would be a significant gain for the code generator. Maybe codegen should just output those tests for data alternatives that contain a recursive use of the data type first - e.g. the cons constructor for lists or the branch constructor for trees? Cheers, Max