[Haskell-beginners] Trouble understanding the type of sequence [Just, Just]

[Lim H.]

Hi everyone,

Sorry if this email disturbs you. I haven't used a developer's mailing list
before so I'm not sure if I'm violating any etiquette. If I do, please
excuse me.

I'm trying to understand the type of sequence [Just, Just]. I can understand
 sequence [Just 1, Just 2] :: Num a => Maybe [a]

because when looking at the type of sequence

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

it is clear that this function takes a collection of monadic values and
return a single monadic value of the collection. Thus, when we call sequence
[Just 1, Just 2] we should get back a Just of [1,2]. Following that train
of thoughts, shouldn't sequence [Just, Just] return a single Just?
Here is the corresponding SO question

http://stackoverflow.com/questions/34244574/trouble-understanding-the-type-of-sequence-just-just

Lim
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