I'd like to remark that the situation with keys is not /quite/ comparable. Even if we don't need to look at the key for inserting into an empty map, it will get forced anyway because IntMaps are spine-strict (in particular, they are strict in their keys.) Edward Excerpts from Johan Tibell's message of Thu Oct 27 12:56:10 -0400 2011:
Hi all,
As discussed in an earlier (accepted) proposal we're adding Data.[Int]Map.{Strict,Lazy} APIs to make it easier for users to get the desired strictness.
There are two subtly different ways these APIs can be strict:
1. All the functions are strict in the value argument, or 2. The structure is strict in the value field.
The difference shows up in functions like insertWith. For example, given
insertWith (\ old _new -> old + 1) k v someMap
should 'v' be evaluated is the key 'k' is already in the map. (Note that the value isn't used in the update function.)
If we go with option 2 it is more difficult for the user to reason about if a given value passed to the API is evaluated or not (as in the above example). We currently use option 1 for keys i.e.
insert k v someMap
is strict in the key even if the map is empty (which wouldn't require us to look at the key).
Thoughts? I'd love to see some design pros/cons of both approaches.
-- Johan