January 2021 - Libraries

Adding partial foldl1' to Foldable?
by Viktor Dukhovni 11 Jan '21

11 Jan '21

Given that Foldable currently has: - foldr and foldr' - foldl and foldl' - foldMap and foldMap' and also has only: - foldr1 - foldl1 it seems natural to ask whether there it should also have a strict variant of at least foldl1, since the non-strict variant has rather limited applicability, and users would/should in most cases want/use the strict `foldl1'` instead. -- Viktor. P.S. I just joined the list today, but noticed that coincidentally, there's already a recent dicussion of Foldable1, which rather overlaps with this question, and perhaps the partial `foldr1` and `foldl1` should be seen as deprecated, once a suitable class of non-empty containers provides total variants. But perhaps on the other hand, given that the partial functions already exist, perhaps adding the strict companions is warranted? I am asking because I am writing some expository prose for Data.Foldable, to go at the bottom of the document, structurally along the lines of what I contributed for Data.Traversable, but with a fairly different focus. The goal is draw careful distinctions between strict recursive and lazy corecursive reductions, explaining their proper usage and typical implementations. The "missing" `foldl1'` was something I ran into while working on part of the writeup.

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can/should Functor have a quantified coercible constraint?
by Carter Schonwald 08 Jan '21

08 Jan '21

Hey everyone! for context, I have some code where I was seeing how far coerce lets me go to avoid doing wrappers for certain codes, i found i had to write the following (mapping an operation over to its newtyped sibling) ``` -- > :t QRA.wither --- forall a b f . Applicative f => (a -> f (Maybe b)) -> RAList a -> f (RAList b) --- wither :: forall a b f . (Applicative f, (forall c d . Coercible c d => Coercible (f c) (f d)) ) => (a -> f (Maybe b)) -> RAList a -> f (RAList b) wither = \f la -> coerce $ QRA.wither f $ coerce la ``` i'd much rather be able to write ``` wither :: forall a b f . (Applicative f) => (a -> f (Maybe b)) -> RAList a -> f (RAList b) wither = \f la -> coerce $ QRA.wither f $ coerce la ``` this seems like it'd be best done via something like changing the functor class definition to ``` class (forall c d . Coercible c d => Coercible (f c) (f d)) ) => Functor f where .. ``` is there any specific reason why this is not feasible? I cant think of a GADT where this wouldn't be totally safe to do (because unlike in foldable, f is in both the domain and co-domain), but maybe i'm not being imaginative enough? look forward to learning what our obstacles are to making this happen for ghc 9.2 :) -Carter

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Proposal: Export Solo from Data.Tuple
by David Feuer 03 Jan '21

03 Jan '21

GHC.Tuple now exports data Solo a = Solo a for use as the canonical lifted unary tuple. I propose to export it from Data.Tuple. David

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