Oops, in the first case it should return Just [], not []
On Oct 8, 2016 9:02 AM, "Joey Eremondi" <joey.eremo...@gmail.com> wrote:
> 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" <duane.john...@gmail.com> 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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