On 2019-01-03 12:43 AM, Ganesh Sittampalam wrote:

If you did have [Maybe Text] you could use the library function

catMaybes :: [Maybe a] -> [a]

to both do the filtering and change the types.

...

But in your case you actually have [Only (Maybe Text)] rather than [Maybe Text] so catMaybes won't work. One option is to use a list comprehension instead:

let fltWDS = [Only a | Only (Just a) <- bd_rows_WDS]

It probably would be helpful to remove the Only wrapper at this stage, so this might be even better:

let fltWDS = [a | Only (Just a) <- bd_rows_WDS]

The way to do it with catMaybes would be to map with fromOnly first:

let fltWDS = catMaybes (map fromOnly bd_rows_WDS)

As Ganesh shows, there's no need for a type annotation after you've done the query, because the compiler can infer the type [Text] from the types of the functions that are used. The annotations are necessary with query only because it's polymorphic and can work with a wide variety of types.