what does the A stand for in sequenceA?
haskell, naming-conventions, traversable
Solution
The "A" stands for `Applicative`, as in the constraint in `sequenceA`'s type:
sequenceA :: (Traversable t, Applicative f) => t (f a) -> f (t a)
That the "A" is there is fruit of a historical accident. Once upon a time, neither `Applicative` nor `Traversable` existed in Haskell. Nonetheless, a function exactly like `sequenceA` already existed -- except that it had a much more specific type:
sequence :: Monad m => [m a] -> m [a]
When `Applicative` and `Traversable` were introduced, the function was generalised from lists to any `Traversable` [1]:
sequence :: (Traversable t, Monad m) => t (m a) -> m (t a)
`sequence`'s `Monad` constraint is unnecessarily restrictive. Back then, however, generalising it further to `Applicative` was not an option. The problem was that until early last year `Applicative` was not a superclass of `Monad` as it is supposed to be, and therefore generalising the signature to `Applicative` would break any use of `sequence` with monads that lacked an `Applicative` instance. That being so, an extra "A" was added to the name of the general version.
[1]: Note, however, that the Prelude continued to carry the list-specific version until quite recently.
Problem
What does `sequenceA` from Traversable stand for? Why is there capital A at the end? I've been learning Haskell for a few months now and this is one of those things that's been bugging me for a while.