I really love Elm, and it challenges the common notion of what it means to be a “healthy” language/ecosystem. It’s a beautiful passion project that can really only be used if you adhere to the vision of its creator, but it’s a really admirable and utopian vision.

The reasons for it not being very popular are pretty clear (slow language evolution, “one way to do anything”, perfectionist design), but it’s an extremely cohesive and friendly language. I’ve never used it for anything serious, but I hope it stays around. Clearly the creator isn’t aiming for widespread adoption, but I hope that the niche it’s created grows nonetheless, because it’s a lovely ecosystem and community.