I'm not sure using and then will work like this in the accumulator, you're
giving it a list of thunks, but andThen tries to chain functions together,
and your functions don't chain like that.
How about this:
forMaybe list = case list of
[] -> []
hm::tm -> hm `andThen` \h ->
(forMaybe tm) `andThen` \t -> Just (h::t)
I apologize for typos, I'm on my phone. This is basically what the forM
function does in Haskell, it works with any type with an andThen, and some
wrapper to put a value in the type (Just in the case of Maybe).
On Oct 8, 2016 8:42 AM, "Duane Johnson" <[email protected]> wrote:
> To avoid the XY problem: I'm aiming to create a function that will take a
> List (Maybe a) and return a Maybe (List a), where the result will be
> Nothing if ANY of the (Maybe a)s are Nothing, otherwise it will be a list
> of just the "a"s.
>
> Here's where I'm getting tripped up in that quest, however: when I
> manually combine a list of numbers (0 through 3) things work fine:
>
> ```elm
> {-| This works -}
> test =
> ((((Just 0) `Maybe.andThen` (\_ -> Just 1)) `Maybe.andThen` (\_ ->
> Just 2)) `Maybe.andThen` (\_ -> Just 3))
> ```
>
> but when I try to use `foldr` and `andThen`, it fails:
>
> ```elm
> {-| This does NOT work -}
> test : Maybe number
> test =
> let
> func =
> Maybe.andThen
>
> acc =
> (Just 0)
>
> list =
> [ (\_ -> Just 1), (\_ -> Just 2), (\_ -> Just 3) ]
> in
> List.foldr func acc list
> ```
>
> I get the error:
>
> ```
> The 3rd argument to function `foldr` is causing a mismatch:
>
> Function `foldr` is expecting the 3rd argument to be:
>
> List (Maybe a)
>
> But it is
>
> List (a -> Maybe number)
> ```
>
> When I change it and remove the anonymous functions in the list (i.e.
> ```list = [ Just 1, Just 2, Just 3 ]```) I get a new error, which I believe
> is leading me down a wrong path (Function `foldr` is expecting the 2nd
> argument to be `a -> Maybe b` But it is `Maybe number`).
>
> Any hints here? I've been hitting my head against the wall for a few
> hours. Any help would be appreciated. (Is this an Elm bug?)
>
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