On 2018-12-18 7:43 AM, Damien Mattei wrote:

i hoped asked could avoid using sort of Null pointer as mentioned in this article:
https://www.lucidchart.com/techblog/2015/08/31/the-worst-mistake-of-computer-science/
if someone else have a simpler solution email me

You can avoid it: make the DNUM column non-NULL in your database schema. That "worst mistake" applies to databases as well as programming languages! :-)

If you have to allow NULL in the database, then you have to handle Nothing in your code. However, a neater way of doing it is to use the maybe function:

forM_ bd_rows_WDS $ \(Only x) ->
      putStrLn $ "x =" ++ maybe "" Text.unpack x

This allows you to provide a default value in your code for when the column is NULL.