import "JavaBuiltin" as Java

@JavaType "java.util.Collection"
data Collection a

@macro
collectionToList :: Collection a -> [a]
collectionToList = Java.unsafeCoerce

singleton :: a -> Collection a
singleton = Java.staticMethod "java.util.Collections.singletonList"

main :: [Integer]
main = collectionToList (singleton 15)
--
[15]