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 Lenses and Traversals to access the inner header value directly and update it.
put $ mail & headers . at header ?~ value
Note that (?~) is just shorthand for \lens value -> lens .~ Just value. The Just is needed to indicate to the at 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 this
do
mail <- get
let oldHeaders = mail ^. headers
put $ (headers .~ (insert header value oldHeaders)) mail
then it's simpler to write that with modify :: MonadState s m => (s -> s) -> m ()
modify (headers . at header ?~ value)
Which, as suggested by Ørjan Johansen in the comments, can be written most pithily as
headers . at header ?= value
~ have monadic state updating versions ending in =, we can combine the ?~ and the modify: headers . at header ?= value
headers . at header . traversed .= value?
at header . traversed .= value and at header ?= value are different. The former (which can also be written as ix header .= value) can only change an existing value, and will do nothing if the value doesn't exist.
You don't usually need to get and put explicitly in the State monad when using lenses. In your case, you can use the operator ?= to update the state directly:
example = do
headers . at header ?= value
You can also modify any lens with a function using %=:
example2 = do
headers %= insert header value
