[Haskell-cafe] handling NULL value in database query with Maybe (or other ...)

[Damien Mattei]

Le 18/12/2018 16:22, Neil Mayhew a écrit :
> 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! :-)

yes :-) but it is not me that create the database,it's a 20 years old
database....
> 
> 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.
> 
> 
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-- 
Damien.Mattei at unice.fr, Damien.Mattei at oca.eu, UNS / OCA / CNRS