Their type system is pretty interesting, and allows for some really cool abilities to parameterize things using types. I'd like to have seen more work done on, effectively, strong typedefs (or whatever $lang wants to call them). However that sort of thing is fairly uncommon so it's hard to hold it against them too much.
The biggest issue, and one they seem unwilling to really address, is that actually using the type system to do anything cool requires you to rely entirely on documentation which may or may not exist (or be up-to-date).
Each type has an entirely implicit interface which must be implemented. There is no syntax to mark which methods must be present for a type. No marker for the method definitions, no block where they must be named, or anything like that. You can't even assume you'll find all the interface methods defined in a single file because they can appear literally anywhere.
Whoever wrote the type has in mind some interface, a minimal set of methods, that must be present for any derived type. There are only two possible ways to determine this. The first is to look to the documentation. Even for the basic types defined by Julia this documentation doesn't seem to exist for all types. I don't have high hopes for most libraries to provide, and keep up to date, this documentation either. This concern gets even greater when considering the focus is largely on scientific computing.
Without up-to-date documentation, the only option is to manually review every file in a library and keep track of the methods defined for the type you're interested in. With multiple dispatch, you can't even get away with just checking the first parameter either. Then you need to look at the definitions for those methods to narrow your list down to the minimal set required. This is not an easy task.
This issue has been brought up before and discussed, but nobody seemed very interested in it. This is a fairly major issue in my view, as it cripples the otherwise very interesting type system. As it stands, it seems to be a fairly complex solution to the issue of getting good compiled code out of a dynamic language. It could be so much more.
The main reason this has not been implemented as a language feature is that people are worried about settling on a design that would be impossible to make fast and concise. It is certainly on the designers' radar, and was discussed specifically at JuliaCon 2017 in Jeff Bezanson's talk.
There are some people who plan to attempt a trait system as a package on Julia 1.0. Perhaps this will be successful and we won't need language changes! Stay tuned.
As an aside, I wouldn't take the lack of action as lack of interest. People are interested, but it wasn't prioritized yet. It will get effort and attention!
First, this extends deep into the core of Julia, and I don't see how a traits package would be involved with that.
Second, and this dovetails with the first issue, this needs to be something that people actually use as a default action. Part of that means ensuring the built-in types make use of this.
Related to all of this, I worry it's too late to really make a meaningful change here. The culture and existing packages are already set without it. Adding the feature as a requirement isn't going to happen anytime soon unless people are willing to continue to break things post 1.0. And it needs to be a requirement or it won't get used except by people that will already provide documentation.
The lack of interest I mentioned mainly came from some github issues which either received little attention, had creators that had a very "maybe it would be nice if" attitude, and responses that were questioning the benefit compared to the cost of implementing the syntax changes.
No, Tim Holy described over dinner how all that was necessary to complete the existing traits packages was method deletion, and that's in v1.0.
But as an external package it still has the issues I described as not being part of the defaults of the culture. Without that it just becomes a nice-to-have that only people serious about writing good quality, usable code are going to use. And these are the people most likely to have good documentation in any case.
As a comparison, consider that the C++ standards body has been working on, debating and serially rejecting the various "C++ concepts" proposals for years now.
If you want to get fancier, yeah, it gets hard. But something as simple as a syntax block that lists the interface methods would be enough for now.
The fancier stuff is already projected so far out to at least 2.0 that I don't understand why a simple working solution wouldn't be desired for now. It would also simplify the work of changing code later to work with 2.0.
I really don't understand how this issue wasn't addressed a long time ago, and why it wasn't a blocker for 1.0.
`methodswith` tells you all methods that been defined on a type. Since it returns an array of methods you can do some more introspection on that, if you're so inclined.
I've also just written a few test cases using the interface I'd like, then worked on methods until my tests pass (which can be just not throwing MethodErrors).