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

[tom.stejskal]

Hi Damien,

the problem is that you are trying to use a type (Maybe), but you should use a data constructor (in this case Just or Nothing) in the pattern match (see the error message).

Something like:

forM_ bd_rows_WDS $ \x ->
  case x of
    Nothing -> putStrLn "NULL"
    Just y -> putStrLn $ Text.unpack y


Regards, Tom


Sent with ProtonMail Secure Email.

‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐
On Tuesday, December 18, 2018 12:11 PM, Damien Mattei <mattei at oca.eu> wrote:

> unfortunately no i'm using Database.MySQL.Simple
> and the doc mention also:
>
> Handling null values
>
> The type of a result tuple will look something like this:
>
> (Text, Int, Int)
>
> Although SQL can accommodate NULL as a value for any of these types,
> Haskell cannot. If your result contains columns that may be NULL, be
> sure that you use Maybe in those positions of of your tuple.
>
> (Text, Maybe Int, Int)
>
> If query encounters a NULL in a row where the corresponding Haskell type
> is not Maybe, it will throw a ResultError exception.
>
> but when i put Maybe in my code like this:
>
> forM_ bd_rows_WDS $ \(Maybe a) ->
>
>       putStrLn $  Text.unpack a
>
>
> i have this error at compilation:
>
> *Main> :load UpdateSidonie
> [1 of 1] Compiling Main ( UpdateSidonie.hs, interpreted )
>
> UpdateSidonie.hs:255:27: error:
> Not in scope: data constructor ‘Maybe’
> Perhaps you meant variable ‘maybe’ (imported from Data.Maybe)
> |
> 255 | forM_ bd_rows_WDS $ \(Maybe a) ->
>
>     |                           ^^^^^
>
>
> Failed, no modules loaded.
>
> Le 18/12/2018 11:59, Paul a écrit :
>
> > If you use Database.Sqlite.Simple library you need to declare returning
> > type as "Maybe XYZ" mandatory., so type will be
> > Only (Maybe String). When I forget about Maybe, I get then same error as
> > you.
> > 18.12.2018 12:16, Damien Mattei wrote:
> >
> > > Hi,
> > > i have this query in SQL used by my Haskell code:
> > >
> > >      let name = "A    20"
> > >
> > >
> > > -- return the list of N°BD from WDS for a given name
> > > let qry_head_WDS = "select DNUM from WDS where DISC = ?" :: Query
> > > putStrLn "before query WDS"
> > > bd_rows_WDS <- query conn qry_head_WDS (Only (name::String))
> > > putStrLn "after query WDS"
> > >
> > >      forM_ bd_rows_WDS $ \\(Only a) ->
> > >        putStrLn $  Text.unpack a
> > >
> > >
> > > works well if there is no NULL values in a database, but if NULL value
> > > for field N°BD i got this exception at runtime:
> > > before query WDS
> > > *** Exception: UnexpectedNull {errSQLType = "String", errHaskellType =
> > > "Text", errFieldName = "DNUM", errMessage = "unexpected null in table
> > > WDS of database sidonie"}
> > > *Main>
> > > i need some code ,perheaps uing Maybe to handle the NULL value in the
> > > field N°BD
> > > i think Haskell have special feature to program this , i read the
> > > article:
> > > https://lukeplant.me.uk/blog/posts/null-pointers-vs-none-vs-maybe/
> > > but for now i have no concrete solution....
> > > Regards,
> > > Damien
> >
> > Haskell-Cafe mailing list
> > To (un)subscribe, modify options or view archives go to:
> > http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe
> > Only members subscribed via the mailman list are allowed to post.
>
> --
>
> Damien.Mattei at unice.fr, Damien.Mattei at oca.eu, UNS / OCA / CNRS
>
> Haskell-Cafe mailing list
> To (un)subscribe, modify options or view archives go to:
> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe
> Only members subscribed via the mailman list are allowed to post.