One way to figure out how to solve this problem is to search Hoogle for the
type of function you’re looking for, in this case: [Maybe a] -> [a]
https://www.haskell.org/hoogle/?hoogle=%5BMaybe+a%5D+-%3E+a
You’ll find that such a function exists in Data.Maybe and it is named
catMaybes. No need to filter out Nothing separately, this function will do
it.
On Wed, Jan 2, 2019 at 20:08 Damien Mattei
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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