Help understanding type signature of flatMap in section 6.1?

[yousefmoazzam]

Hi 🙂 Many thanks to the author for creating this tutorial, it's been very useful for me in trying to grasp some of the typeclasses in haskell!

I was hoping I could get some help understanding something? I was trying to do the exercise where the reader is asked to provide an implementation of bindExceptT in section 6.2, so I decided to revisit section 6.1 where flatMap was introduced.

The type signature given for flatMap is the following:

learn-haskell-blog-generator/src/06-errors_and_files/01-either.md

Lines 350 to 351 in a0dac9c

	flatMap :: (a -> Either e b) -> Either a -> Either b
	flatMap func val = flatten (fmap func val)

and I was having trouble understanding the second arg type and the return type; namely, that they both have only one type parameter filled-in for the Either type constructor.

I was trying to reason through it in my head, and I was half-wondering if this should maybe be:

flatMap :: (a -> Either e b) -> Either e a -> Either e b
flatMap func val = flatten (fmap func val)

One reason I thought this might be the case is that this would make the kind of both of them be * rather than * -> *?

I was also thinking that, since flatMap is doing:

• the "lifting" of a function via fmap
• applying the lifted function to a value of the appropriate type
• applying flatten

that the return type of flatMap should ultimately be the same as doing fmap and then flatten, and flatten has the following type signature:

learn-haskell-blog-generator/src/06-errors_and_files/01-either.md

Lines 325 to 329 in a0dac9c

	flatten :: Either e (Either e a) -> Either e a
	flatten e =
	case e of
	Left l -> Left l
	Right x -> x

whose return type is of kind *?

The above reasoning kinda makes sense to me, but I'm not at all confident in this... 😅 Would anyone be able to help me understand this?

Thanks again!

[soupi]

No, you are right. The text is wrong. This should be as you said:

flatMap :: (a -> Either e b) -> Either e a -> Either e b
flatMap func val = flatten (fmap func val)

Thanks for spotting and reporting. Will fix this momentarily.

[yousefmoazzam]

Ah ok cool, thanks for the quick reply and clarification :)