Currently, unsafeInterleaveST is considerably less safe than unsafeInterleaveIO. Specifically, very careful discipline is required to make code using it thread-safe. This means that unsafeInterleaveST actually behaves like unsafeDupableInterleaveIO. See Edward Kmett's blog post from several years ago [*] for details. I propose that we should 1. Make unsafeInterleaveST behave only as unsafely as unsafeInterleaveIO, and 2. Add a new unsafeDupableInterleaveST function implementing the current unsafeInterleaveST behavior. The alternative would be to leave unsafeInterleaveST alone and add a new unsafeNonDupableInterleaveST, but that seems like an absolutely terrible mismatch of names. Since any currently-correct code using unsafeInterleaveST will continue to work (albeit more slowly), I don't think that's justified. Thanks, David [*] https://www.schoolofhaskell.com/school/to-infinity-and-beyond/older-but-stil... f