I have a couple of snippets which feel like they're doing the same thing, but I'm not entirely convinced there is a generalised construct to handle them both. In one place, I have
ensure :: (String -> Bool) -> String -> String
ensure p x =
if p x then
x
else
""
This might in use look something like
ensure (/= "kim") "alex" -- returns "alex"
ensure (/= "kim") "kim" -- returns ""
in another, I have the very similar
ensure :: (a -> Bool) -> Maybe a -> Maybe a
ensure p maybeX = do
x <- maybeX
if p x then
Just x
else
Nothing
This would instead look something like
ensure even 6 -- returns Just 6
ensure even 11 -- returns Nothing
Both are checking whether a value is correct according to some predicate, and if it's not they're returning a default "empty"-looking value. There is a slight difference though – which means the second function could be rewritten as
ensure :: (Maybe a -> Bool) -> Maybe a -> Maybe a
ensure p maybeX =
if p x then
x
else
Nothing
to make them more similar, putting the responsibility of "unwrapping" the Maybe on the predicate. With this new definition, both functions would fall under
ensure :: Alternative f => (f a -> Bool) -> f a -> f a
ensure p x =
bool x empty (p x)
So, my question is,
Does this bool x empty (p x) exist in some form so I don't have to implement this function myself? The problem with inlining bool x empty (p x) is that in my case, both p and x are quite long.
Below are suggestions from the comments. One using Monoid, by 9000:
ensure :: Monoid a => (a -> Bool) -> a -> a
ensure p a = if p a then a else mempty
Another using MonadPlus, by user3237465:
ensure :: MonadPlus m => (a -> Bool) -> a -> m a
ensure p = mfilter p . return
And a variant of the second one that only requires Alternative, by Daniel Wagner:
ensure :: Alternative f => (a -> Bool) -> a -> f a
ensure p x = x <$ guard (p x)