August 2019 - Libraries

Proposal: add indexed map and traverse to Data.List
by David Feuer 17 Aug '19

17 Aug '19

mapWithIndex :: (Int -> a -> b) -> [a] -> [b] mapWithIndex f = zipWith f [0..] traverseWithIndex :: Applicative f => (Int -> a -> f b) -> [a] -> f [b] traverseWithIndex f = sequenceA . mapWithIndex The real implementation of mapWithIndex (and therefore of traverseWithIndex) can be a "good consumer" for list fusion. mapWithIndex can be a "good producer" as well (which the naive implementation already accomplishes). Similar functions (with these or similar names) are already common in packages like vector, containers, unordered-containers, and primitive. A more general function would merge zipping with unfolding: zipWithUnfoldr :: (a -> b -> c) -> (s -> Maybe (b, s)) -> [a] -> s -> [c] zipWithUnfoldr f g as s = zipWith f as (unfoldr g s) But this doesn't seem like the friendliest or most obvious user interface, so I am not proposing to add it to base.

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Add instance MonadFail (f :*: g)
by Dannyu NDos 15 Aug '19

15 Aug '19

instance (MonadFail f, MonadFail g) => MonadFail (f :*: g) where fail s = fail s :*: fail s

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Make sameNat proxy-polymorphic
by Andrew Lelechenko 15 Aug '19

15 Aug '19

Hi all, Currently base:GHC.TypeNats.sameNat is implemented as follows: sameNat :: (KnownNat a, KnownNat b) => Proxy a -> Proxy b -> Maybe (a :~: b) sameNat x y | natVal x == natVal y = Just (unsafeCoerce Refl) | otherwise = Nothing There are applications, when more flexibility is desirable. I propose to change sameNat's type to sameNat :: (KnownNat a, KnownNat b) => proxy1 a -> proxy2 b -> Maybe (a :~: b) It does not require any changes in the implementation, because natVal is already "proxy-polymorphic": natVal :: forall n proxy. KnownNat n => proxy n -> Natural Best regards, Andrew

4 3

indexM or (!?)-style accessor for arrays?
by Zemyla 13 Aug '19

13 Aug '19

This is an issue I originally proposed on GHC Trac, but I'm posting it here because (I think) the Data.Array package is under the purview of the Libraries committee. The vector package has indexM and its cousins so that a user can be strict in the array without necessarily being strict in the value retrieved from that array. Arrays don't have that sort of thing, meaning that anything you do that takes an array will necessarily leave references to the array unless you force the whole thing, and that's not only inefficient, it's untenable for general libraries. What I'm thinking is that the IArray class should have a function like unsafeAtM :: (Array a e, Ix i, Applicative m) => a i e -> Int -> m e For compatibility with older code that wouldn't necessarily define this but would define unsafeAt, we'd have the default implementation unsafeAtM a !n = pure (unsafeAt a n) Also, you could have unsafeAt defined in terms of unsafeAtM, so the minimal implementation could require only one of them: unsafeAt = (coerce :: (a i e -> Int -> Identity e) -> a i e -> Int -> e) unsafeAtM Also, (!?) would be a "safe" indexing tool for arrays, which would incidentally also force the array without forcing the value inside. You would have (!?) :: (Array a e, Ix i) => a i e -> i -> Maybe e (!?) a e = case bounds a of p@(l, u) -> case inRange p e of True -> unsafeAtM arr $ unsafeIndex p e False -> Nothing

2 4

Add instance Eq1 Complex, Show1 Complex, and Read1 Complex
by Dannyu NDos 11 Aug '19

11 Aug '19

instance Eq1 Complex where liftEq f (x :+ y) (u :+ v) = f x u && f y v instance Show1 Complex where liftShowsPrec sp _ p (x :+ y) = showParen (p >= 6) (sp p x . showString " :+ " . sp p y) instance Read1 Complex where liftReadPrec rp _ = parens $ prec 6 $ do x <- step rp Symbol ":+" <- lexP y <- step rp return (x :+ y) liftReadList = liftReadListDefault liftReadListPrec = liftReadListPrecDefault

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Request for comment on new package
by David Feuer 09 Aug '19

09 Aug '19

I've put together a small package for lazifying record types. There's a "classy" version in Data.Lazify and a Generic-only version in Data.Lazify.Generic. A few examples are below. I'd love to hear comments on what looks good and what should be changed before I make the first release. As usual, names are the hardest part. Note especially that the operator ($~) is defined in *both* Data.Lazify and Data.Lazify.Generic, and that no operator corresponds to genericLazify. Hrmm mumble. lazify (1, 2) = (1, 2) lazify undefined = (_|_, _|_, _|_) lazify undefined = Sum (_|_, _|_, _|_) genericLazify (1,2) = (1,2) genericLazify undefined = (_|_, _|_, _|_) genericLazify (Sum (MyCon x y)) = ... oops, MyType isn't an instance of Lazifiable lazifyGeneric (1,2) = (1,2) lazifyGeneric undefined = (_|_, _|_, _|_) lazifyGeneric (Sum (MyCon x y)) = Sum (MyCon x y) Thanks in advance, David

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Add instance Alternative (Either e)
by Dannyu NDos 08 Aug '19

08 Aug '19

instance Monoid e => Alternative (Either e) where empty = left mempty Left x <|> Left y = Left (x <> y) Left _ <|> y = y x <|> _ = x Leftmost Right is returned for sake of consistence with Semigroup instance.

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Re: Add instance Monoid ((f :+: g) a)
by Dannyu NDos 06 Aug '19

06 Aug '19

Ah, sorry. (:+:) is not an Applicative. It's still possible to make instance Monoid: instance Semigroup (f a) => Semigroup ((f :+: g) a) where InL x <> InL y = InL (x <> y) InL _ <> y = y x <> _ = x instance Monoid (f a) => Monoid ((f :+: g) a) where mempty = InL mempty

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Add instance Alternative (f :+: g)
by Dannyu NDos 06 Aug '19

06 Aug '19

instance Alternative f => Alternative (f :+: g) where empty = InL empty InL x <|> InL y = InL (x <|> y) InL _ <|> y = y x <|> _ = x This is analogous to the proposed Alternative Either instance.

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foldrM definition
by Dannyu NDos 06 Aug '19

06 Aug '19

Wouldn't it be friendlier to list structures if foldl' was used instead of foldl?

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