[Haskell-cafe] Maybe type filtered

[Ivan Lazar Miljenovic]

That's because you still have "Just a" values. Try mapMaybe from Data.Maybe
instead.


On mobile; please excuse any tpyos.

On Thu, 3 Jan. 2019, 1:29 pm Damien Mattei <damien.mattei at gmail.com wrote:

> i repost because it seems that the first post of the year in haskell cafe
> ailing list has been lost! i cant't see in the archive of january neither
> december....
> so here it is again:
>
> On Tue, Jan 1, 2019 at 10:25 AM Damien Mattei <damien.mattei at gmail.com>
> wrote:
>
>> i had filtered a [Maybe Text] type to remove Nothing from the list and
>> now i want to put the result in a [Text] type but the compiler complains
>> about the incompatible type :
>>
>>  (bd_rows_WDS :: [Only (Maybe Text)]) <- query conn qry_head_WDS (Only
>> (name::String))
>>
>> -- remove the records having N°BD NULL
>>     let fltWDS :: [Only Text] = Prelude.filter (\(Only a) ->
>>                                     case a of
>>                                       Nothing -> False
>>                                       Just a -> True)
>>                  bd_rows_WDS
>>
>> Prelude> :load UpdateSidonie
>> [1 of 1] Compiling Main             ( UpdateSidonie.hs, interpreted )
>>
>> UpdateSidonie.hs:282:33: error:
>>     • Couldn't match type ‘Maybe Text’ with ‘Text’
>>       Expected type: [Only Text]
>>         Actual type: [Only (Maybe Text)]
>>     • In the expression:
>>         Prelude.filter
>>           (\ (Only a)
>>              -> case a of
>>                   Nothing -> False
>>                   Just a -> True)
>>           bd_rows_WDS
>>       In a pattern binding:
>>         fltWDS :: [Only Text]
>>           = Prelude.filter
>>               (\ (Only a)
>>                  -> case a of
>>                       Nothing -> False
>>                       Just a -> True)
>>               bd_rows_WDS
>>       In the expression:
>>         do conn <- connect
>>                      defaultConnectInfo
>>                        {connectHost = "moita", connectUser = "mattei",
>>                         connectPassword = "sidonie2", connectDatabase =
>> "sidonie"}
>>            (rows :: [(Text, Double)]) <- query_
>>                                            conn
>>                                            "SELECT Nom,distance FROM
>> AngularDistance WHERE distance > 0.000278"
>>            (names :: [Only Text]) <- query_
>>                                        conn
>>                                        "SELECT Nom FROM AngularDistance
>> WHERE distance > 0.000278"
>>            let resLstNames = Prelude.map fromOnly names
>>            ....
>>     |
>> 282 |     let fltWDS :: [Only Text] = Prelude.filter (\(Only a) ->
>>     |                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^...
>> Failed, no modules loaded.
>>
>> how can id do the type conversion,now i'm sure there is no more Nothing
>> values?
>>
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