sequence in haskell

[metric-space]

the type signature of sequence in Haskell is

sequence :: (Monad m, Traversable t) => t (m a) -> m (t a)

so I could have sequence [Left 4, Right 5] => Left 4

and sequence [Right 6, Right 9] => Right [6,9]

Is there an equivalent in sanctuary? I've been scanning libraries that implement the fantasy-land spec, none of them seem to have this (so far)

Edit : I understand it can be implemented as such (atleast for my task at hand)
x => S.reduce(S.lift2(S.C(S.append)),S.Right([]),x), was wondering why something as such is missing

[davidchambers]

v0.11.1 provides:

• Maybe#sequence :: Applicative f => Maybe (f a) ~> (a -> f a) -> f (Maybe a); and
• Either#sequence :: Applicative f => Either a (f b) ~> (b -> f b) -> f (Either a b).

v0.12.0 will provide fantasy-land/traverse methods instead (for compatibility with version 2 of the Fantasy Land spec) and polymorphic traverse and sequence functions which wrap Z.traverse and Z.sequence respectively.

Does this answer your question, @functor-soup?

[safareli]

so I could have sequence [Left 4, Right 5] => Left 4

I think you would get Left [4]

[joneshf]

λ: :t sequenceA [Left 4, Right 5]
sequenceA [Left 4, Right 5] :: (Num a1, Num a) => Either a1 [a]

[metric-space]

@davidchambers Hmm I do remember looking at that for quite some time, thought it was something else. I see I am wrong. How would you use const b = [S.Left(4), S.Right(5)] to get Left(4) using sanctuary'ssequence?

@safareli I believe @joneshf's comment clarifies that the answer would be Left 4 because the type signature highlights that pretty clearly

[safareli]

@functor-soup correct

[davidchambers]

Here are the Sanctuary equivalents of your Haskell expressions, @functor-soup, on the dc-fantasy-land branch:

> S.sequence
sequence :: (Applicative f, Traversable t) => TypeRep f -> t (f b) -> f (t b)

> S.sequence(S.Either, [S.Left(4), S.Right(5)])
Left(4)

> S.sequence(S.Either, [S.Right(6), S.Right(9)])
Right([6, 9])

I'll close this issue as I believe we've answered your question. :)