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