On Mon, Jan 20, 2020 at 10:03:16AM +0000, PICCA Frederic-Emmanuel wrote:
chunk :: (Num n, Ord n) => n -> [Chunk n a] -> [[Chunk n a]] chunk target = go target target where go _ _ [] = [] go tgt gap [x] = golast tgt gap x go tgt gap ~(x:xs) = let gap' = gap - cweight x in if | gap' > 0 -> cons1 x $ go tgt gap' xs | gap' == 0 -> [x] : go tgt tgt xs | (x1, x2) <- csplit x gap -> [x1] : go tgt tgt (x2 : xs)
cons1 !x ~(c:cs) = (x : c) : cs
golast tgt gap x = if | cweight x <= gap -> [x] : [] | (x1, x2) <- csplit x gap -> [x1] : golast tgt tgt x2
I've tried both GHC 8.6.5 and GHC 8.8.1, and get no warnings with either "-O -Wall" or "-O2 -Wall" and the below (my original names, which look equivalent). chunk :: (Num n, Ord n) => n -> [W n a] -> [[W n a]] chunk target = go target target where go _ _ [] = [] go tgt gap [x] = golast tgt gap x -- XXX: Try dropping the "~" on the next line it should be redundant go tgt gap ~(x:xs) = let gap' = gap - weight x in if | gap' > 0 -> cons1 x $ go tgt gap' xs | gap' == 0 -> [x] : go tgt tgt xs | (x1, x2) <- wsplit x gap -> [x1] : go tgt tgt (x2 : xs) cons1 !x ~(c:cs) = (x : c) : cs golast tgt gap x = if | weight x <= gap -> [x] : [] | (x1, x2) <- wsplit x gap -> [x1] : golast tgt tgt x2
But when compiling I have these warning
src/Hkl/Projects/Sixs.hs:(59,5)-(65,69): warning: [-Wincomplete-uni-patterns] Pattern match(es) are non-exhaustive In a pattern binding: Patterns not matched: [] | 59 | go _ _ [] = [] | ^^^^^^^^^^^^^^^^^^^^^^^...
I do not understand this one
See the XXX comment I added above, with that and also "-Wincomplete-uni-patterns" in addition to "-Wall", I no longer get that warning.
src/Hkl/Projects/Sixs.hs:67:5-35: warning: [-Wincomplete-uni-patterns] Pattern match(es) are non-exhaustive In a pattern binding: Patterns not matched: [] | 67 | cons1 !x ~(c:cs) = (x : c) : cs | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
If this is something which can not be reach, is it possible to explain to ghc how to avoid these warning ?
However, with "-Wincomplete-uni-patterns" I still get this one, but the irrefutable binding is there for a reason, stricness in the second argument has undesirable performance implications, we know that the list won't be empty. So perhaps compile this code in a module that does not have that warning turned on. It is not too difficult to rewrite "chunk" so that it operates on (type-level) non-empty lists, but perhaps easier to just ignore or disable the warning. -- Viktor.