Also, I've found Haskell appropriate for some one-off tasks over the years, e.g.
- Extracting a load of cross-referenced data from a huge XML file. I tried a few of our "common" languages/systems, but they all ran out of memory. Haskell let me quickly write something efficient-enough. Not sure if that's ever been used since (if so then it's definitely tech debt).
- Testing a new system matched certain behaviours of the system it was replacing. This was a one-person task, and was thrown away once the old system was replaced; so no tech debt. In fact, this was at a PHP shop :)