Around a year ago, there was a lively debate about the type invariance of the immutable Set in Scala. Dogpile argumentation on a subject far outside the popular interest is of course thrilling in itself, but the topic also provides a nice focal point for exploring and clarifying some important aspects of the Scala type system.

We recall that Scala collections (and other higher kinded classes) can be invariant, covariant or contravariant in their type parameters, corresponding repectively to declarations as class Whatever[A], class Whatever[+A] or class Whatever[-A].

• In the case of covariance, a Whatever[B] will …

Some time after my recent fiddles with IMDB, I read an interesting article about using a perceptron to classify words as parts of speech based on features that precede them in text. It's all done in python or some such sh*t, but whatever. Still very cool. Since I had all of this IMDB data accumulated in Mongo, I thought I would try to play with it, and the idea I had was to predict metacritic scores from the actors that appeared in each film. In retrospect, it's far from clear that such a prediction can be made and especialy …

I am broadly sympathetic to view that scalable systems must be built with statically typed langages, for reasons outlined in this wonderful screed, and, until recently, that has made it difficult for me to recommend clojure for institutional use.

With the introduction of core.typed, that has changed. The author has says that core.typed is now production-ready, and I agree. It's not perfect, but it will find bugs in your code without breaking it or causing performance problems. It's also pretty cool, and in many ways more expressive than type declarations in "normal" statically typed languages.

That said, the …