[the_af]

Agreed, Elm in these two examples is way nicer! It doesn't add information, but it presents it in a beginner-friendly way, which is commendable.

However, I'm left wondering... Elm seems to be using hardcoded knowledge about commonly used types such as strings and numbers in order to improve its error messages (I wonder why Haskell doesn't, by the way). What happens in Elm when an error occurs with more complex, user-defined types and operations?

[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

[masklinn]

In the "+" case, only the final hint is type-specific. The first error is kind-of type-specific but only distinguishes functions and non-functions.