FWIW, as someone who often interns strings in her programs, I'd suggest *not* naming it "intern" (or variations thereon). One issue is the "and store it if it's not there yet" issue already mentioned. But another —and imo more important— issue is that one of the main goals of interning things is so that you can get a fast equality check. In general whenever we intern things we're going to return some token, which supports fast equality, and takes up as little memory as possible, but which can later on be cashed in for the original (shared) value if we actually need it. In impure languages we can use the pointer to the object as our token, but we can't do that (without serious grossness) in Haskell so we'd return some other token like an integer, perhaps with a newtype wrapper to keep track of its providence so we can avoid mixing up tokens from different intern tables. I already have an implementation of intern tables for ByteString in my local libraries; it'd be trivial to extend that to intern tables for String or Text or anything else supporting either (a) equality and hashability, or (b) orderability. IMO, if users want an intern table then they should ask for one, they shouldn't use some ad hoc combination of other data types which obfuscates the true purpose. Adding this to containers should be proposed on a separate thread, but I'm entirely willing to do it if people actually want it -- Live well, ~wren