On 7/12/07, Dan Weston
Now that I mention it, the base case is much less often called than the recursive case, so I would in hindsight flip the order of the two (mutually exclusive) partial function definitions:
unique :: Eq a => [a] -> [a] unique (x:xs) = x : (unique . filter (/= x) $ xs) unique [] = []
This should be marginally more efficient. I doubt that GHC would automatically detect that a) they are mutually exclusive and b) the second is called less often than the first!
Of course GHC detects that the cases are mutually exclusive. The code above desugars into a single definition of unique with a case expression on its argument. In Core, case expressions have at most one alternative for each data constructor of the type of the scrutinee, and since each alternative corresponds to a different top-level data constructor, alternatives are mutually exclusive. As for point (b), it hardly matters, since GHC will rearrange case alternatives at will (and I don't think the order of alternatives has any operational meaning anyway.) If you want to see this for yourself, try running GHC with the -ddump-simpl flag on a simple example (like the one above). Cheers, Tim -- Tim Chevalier* catamorphism.org *Often in error, never in doubt "What doesn't kill you, makes you stronger -- or puts you on a talk show." --Carrie Fisher