I find myself using this pattern often:
do
let oldHeaders = mail ^. headers
put $ (headers .~ (insert header value oldHeaders)) mail
which seems like the kind of thing Control.Lens should be able to do, but I guess I just haven't found the right operator yet. Is there a better way? Also, is there anything else I should be doing differently in this code?
You can use a chain of
Lens
es andTraversal
s to access the inner header value directly and update it.Note that
(?~)
is just shorthand for\lens value -> lens .~ Just value
. TheJust
is needed to indicate to theat
lens that we want to insert a value if it doesn't exist already.If
mail
in the first line comes from the state monad like thisthen it's simpler to write that with
modify :: MonadState s m => (s -> s) -> m ()
Which, as suggested by Ørjan Johansen in the comments, can be written most pithily as