I have a list comprehension in Haskell that specifies a predicate on a Maybe type:
[x | x <- listOfMaybes, isJust(f y), x == fromJust(f y)]
is there a way to simplify this expression?
2 Answers
If you simply want to filter out all the Nothings, shang's answer gives the most concise solution. However, it can be done easily in a list comprehension:
[x | Just x <- f y] 
4 Comments
dfeuer
You might want to explain that this is essentially
flip concatMap (f y) $ \result -> case result of {Just x -> [x]; {-Implicitly added!-} _ -> []}
Stephan
Right, I find it easier to just learn that pattern matching in list comprehensions is perfectly valid and automatically filters out non-matching cases. Your desugaring is not exactly adding clarity, I find :-)
JonnyRaa
interesting that the behaviour is different here from pattern matching in do notation where
Just x <- f y might errorStephan
It wouldn't be different to do notation, because it's the monad instance of List that controls how pattern matching errors are handled. And in List, error simply is ignored. This might have changed recently due to the MonadFail proposal, I'm not sure.
You can simply use catMaybes (from Data.Maybe) to filter out all Nothing values.
<-, this is not a list comprehension and I doubt it compiles.x==fromJust(f y)in the list comprehension isn't necessary. Conditioning onisJust(f y)does the job.