小编Dam*_*tei的帖子

我已经过滤了[Maybe Text]类型以从列表中删除Nothing,现在我想将结果放在[Text]类型中,但编译器抱怨不兼容的类型:

(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) …