Using Lens in Haskell to modify values

Question

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?

Safe

Answer 1

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

Unroll

Exactly what I was looking for! Now I just need to step through it so I make sure I understand it. – Safe

(Tried to make this an edit but was rejected:) Remembering that many of the pure setter operators ending in ~ have monadic state updating versions ending in =, we can combine the ?~ and the modify: headers . at header ?= value – Ensemble

Couldn't you just use headers . at header . traversed .= value? – Randee

No -- 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. – Kinakinabalu

@ØrjanJohansen I added your suggestion with a note. – Unroll

@GabrielGonzalez I've been bitten by the difference in semantics you bring up a few times. It's somewhat problematic that both typecheck. – Unroll

Answer 2

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

Decent