How to convert `[[Either String c]]` into `Either String [[c]]`?

Question

Say I have two functions:

b2c :: B -> Either String C
a2bs :: A -> [[B]]

How to make the following a2cs function using b2c and a2bs such that if there is any Left value in the [[Either String c]] then Either String [[c]] should be a Left value?

a2cs :: A -> Either String [[C]]

Answer 1

I would write this:

a2cs = traverse (traverse b2c) . a2bs

Although this doesn't use exactly your proposed implementation strategy of first producing a [[Either String C]] and then flattening that to Either String [[C]], it has the advantage that it requires only one pass over the nested lists rather than the two required by your strategy.