FromForm and ToForm instances for (,) and Either

[wraithm]

Does it make sense to have:

instance (FromForm a, FromForm b) => FromForm (a, b) where
    fromForm f = (,) <$> fromForm f <*> fromForm f

and ditto for Either?

[fizruk]

@wraithm not sure, do you have a use case in mind?

[wraithm]

@fizruk

The specific example is that I have a form for creating groups of users, and then I have a form for creating users. For certain kinds of groups, I need a form with a "root" user for that group, so I just use a tuple of the basic group form and the new user form for those kinds of group forms.

So, I have two forms on two separate pages, and then I have a third form that is basically just both of those two forms on the same form. Of course, I could make a new data type with a new instance, but it was very convenient to use a tuple, and I was surprised to find that this instance didn't already exist. Also, you could have an Either version of this as well, but the tuple case is what I used for my specific problem here.

[phadej]

Question: can corresponding ToForm be written? If it can, then I don't see downsides.

[wraithm]

Yes, you can definitely write a ToForm instance for both (,) and Either. The semantics of toForm after fromForm and vice versa would maybe be a little weird, but I don't think that matters very much.

…

On Mon, Apr 8, 2019, 15:45 Oleg Grenrus ***@***.***> wrote: Question: can corresponding ToForm be written? If it can, then I don't see downsides. — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub <#91 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/ABcV0SegJ6vJZS5Btoyn-y_yoErbISt3ks5ve6pbgaJpZM4cd41c> .

[phadej]

I think we can assume (and mention that in instance haddocks) that forms "read/write" disjoint field sets.