Add Map.insert_with about base HOT 4 CLOSED

Map.update and Map.change do indeed seem to be in the same neighborhood. My first instinct is to think that this doesn't provide enough unique value to be worth adding. A good way of arguing the other side would be to provide some examples of code that would be made cleaner and nicer with this function, as opposed to change or update.

from base.

It's mostly a generalisation of incr (which exists in Hashtbl but not in Map) that let's you specify the combining operator. It's also than update because it only mentions the value-to-be-inserted once.

-      let amount = entry.amount in
-      M.update per_category ~key:c (function None -> amount | Some old -> old + amount)
+      M.update_with per_category ~key:c ~data:entry.amount ~f:(+)

I actually haven't found many use-cases besides insert_with ~f:(+) or insert_with ~f:List.append. Feel free to close if you think it doesn't provide enough unique value.

from base.

Yeah, my tendency is to not think it's worth adding a special function to Map for this.

FWIW, maybe another nice approach would be to define a function like:

val z f x opt =
   match opt with
   | None -> x
   | Some x' -> f x' x

And then you could do:

M.update per_category ~key:c (z (+) x)

But one would need a better name than z. But with an appropriate name, I could imagine adding such a thing to Option.

from base.

But one would need a better name than z. But with an appropriate name, I could imagine adding such a thing to Option.

Interesting. Looks like Option.fold:

M.update per_category c (Option.fold ~init:entry.amount ~f:(+))

I think this works for me. Cheers!

from base.