drop, but also because strings in Haskell are actually lists of characters. Anyway, here it is. Compare with the F# version.
rot13 s = map rotchar s
where rotchar c = maybe '#' id (lookup c transp)
transp = zip letters ((drop 13 letters) ++ (take 13 letters))
letters = ['a' .. 'z']
And now for the use case:
*Rot13> rot13 "feijoada"