On 11/10/2014 04:24 AM, Donn Cave wrote:
quoth Raphaël_Mongeau
This : pv a = [t | Just t <- pvc a] is strange, can we really do pattern matching inside list comprehension?
Sure, but from a list - I'm sorry, I meant "map pvc a", not "pvc a".
pv a = [t | Just t <- map pvc a] where pvc A = Just 'A' pvc B = Just 'B' pvc _ = Nothing
You know, the pattern match in the list comprehension is just what I wanted it for in the first place - remember ['A' | A <- s] ? That's OK, it just isn't useful because I can do this for only one target.
... and I think is more clear with a lambdaCase.
lambdaCase is a great thing, but given that it's hardly any different
pv l = catMaybes $ flip map l $ \case A -> Just 'A' B -> Just 'B' _ -> Nothing
from
pv l = catMaybes $ map pvc l where pvc A = Just 'A' pvc B = Just 'B' pvc _ = Nothing
catMaybes . map f = mapMaybe f Also I wonder if laziness saves us here: in the original program we effectively do map and filter at the same time. If we were to take (catMaybes . map f) with strict evaluation then we'd be traversing twice: once to map and once to catMaybes… Just something to think about, I think performance would be no worse anyway, at least not by much.
... in this case I don't think we're desperate enough to use a nonstandard extension.
I wouldn't worry about using a ‘non-standard’ extension: you're probably not going to stick to H98 or H2010 in non-trivial programs either way. LambaCase is just a trivially expandable sugar anyway, modulo clean identifier name. -- Mateusz K.