[IanCal]

It looks like it's only the specific hint at the bottom that requires any custom knowledge about the types themselves.

If you're interested, there were a couple more examples shown elsewhere in this thread:

http://elm-lang.org/blog/compiler-errors-for-humans (an older version it seems)

https://twitter.com/st58/status/732908457217531904

https://twitter.com/GregorySchier/status/732830868562182144

Something I've just found is that the errors can also be output in a nice machine readable format, so the suggested fixes can be read in by your editor plugin.

And finally, this is wonderful, a specific repo for code that causes error messages and a request for people to submit error messages that are confusing: https://github.com/elm-lang/error-message-catalog