foo :: [Bar] -> [Baz]
foo xs = if (any f xs) then  else [Baz 4]
This can be replaced with a neater list comprehension
[Baz 4 | any f xs]
If the right hand side of the list comprehension evaluation to false, then you get the empty list, else you get the single item in the list.
Neat, though I've no idea whether it's idiomatic Haskell!