How can I add something to a list when a condition is true and do nothing otherwise in Haskell?-CodePudding

I was hoping to find a better way to solve a problem I've encountered in Haskell.

Given a list and a condition create a new list with only elements which satisfy the condition. Below is a solution I used. Is there a better alternative which doesn't involve maybes?

eg :: (Eq  a, Num  a) => (a -> Bool) -> [a] -> [a]
eg cond i = catMaybes (map (\x-> if cond x then Just x else Nothing) i)

CodePudding user response：

Your eg is equivalent to filter :: (a -> Bool) -> [a] -> [a]. Indeed, you can filter with:

filter (\x -> some_condition x) my_list

If some_condition is for example a simple a -> Bool function, this is equivalent to:

filter some_condition my_list

Neither your implementation of eg nor the one with filter require the Eq a and Num a type constraints by the way: one can simply use guards or pattern match on the outcome of cond x.

filter is implemented with explicit recursion [src]:

filter :: (a -> Bool) -> [a] -> [a]
filter _pred []    = []
filter pred (x:xs)
  | pred x         = x : filter pred xs
  | otherwise      = filter pred xs

Here for an empty list it thus returns the empty list, and when the list is not empty, it will only prepend x if pred x is satisfied.