Diagonalization/ dupe for monads and tuples?

Rob Rix rob.rix at me.com
Wed Sep 23 12:10:28 UTC 2020

TL;DR: maybe we can think of this as combining disjoint empty & non-empty cases of folds.

Every time I’ve wanted to define something like this I’ve found myself dissatisfied because of this asymmetry.

Looking at it from another angle, I frequently encounter a vaguely similar situation with folds: given some collection which can be empty or non-, I want to fold over it with a function if non-empty, or return a default value if empty:

foldr1OrDefault :: Foldable t => (a -> a -> a) -> a -> t a -> a
foldr1OrDefault f z t
  | null t    = z
  | otherwise = foldr1 f t

This is a bit like Control.Lens.Fold.foldBy in lens, except that the combining function f does not receive z at any point. Quite useful when you have a Semigroup but not a Monoid, or when you otherwise want to treat the empty case specially.

fromMaybe is a special case of this function, without the conceit of pretending that Maybe can hold multiple values. I’ve sometimes involved Maybe in this construction for just that reason when e.g. a is a Semigroup:

fold1Maybe :: (Foldable t, Semigroup a) => t a -> Maybe a
fold1Maybe = foldMap Just

fold1OrDefault :: (Foldable t, Semigroup a) => a -> t a -> a
fold1OrDefault z = fromMaybe z . foldMap Just

(All of these names are intentionally bad because I don’t want to propose adding these functions anywhere just yet.)

For Either (and These for that matter), a full generalization also has to deal with the extra information in the null case, i.e. Either b a would need (b -> a) instead of a:

fromEither :: (b -> a) -> Either b a -> a
fromEither f = either f id

I’m surprised about once every six months that this doesn’t exist (and then surprised again when consulting the types that fromLeft/fromRight aren’t either this *or* analogous to fromJust).

All of which is to say, I think I would prefer to have the generality of this fromEither to the a -> Either a a -> a version, but it’d be nice to figure out a way to see it as a fold, too. Maybe the Bifoldable1 instance for Either has something to contribute here?


> On Sep 17, 2020, at 1:13 PM, Philip Hazelden <philip.hazelden at gmail.com> wrote:
> On Thu, Sep 17, 2020 at 5:59 PM Jon Purdy <evincarofautumn at gmail.com <mailto:evincarofautumn at gmail.com>> wrote:
> { fromMaybe, fromLeft, fromRight, fromEither, fromThese, fromDynamic, … } extracts “the” value out of a { Maybe, These, Dynamic, … } with a suitable default if “it” isn’t present.
> Hm. A weird thing about fromEither compared to (I think) all the rest of these, is that the type of the Either isn't fully general. That is, fromLeft and fromRight take an `Either a b`; fromThese takes a `These a b`. But the proposed `fromEither` takes an `Either a a`.
> This maybe isn't a big deal. Just seemed worth noting.
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