I recommend people study APL / J / K for the same reason: It has the right primitives that make everything expressible as maps, folds, filters, scans, ranges, and stuff like that.
Even more so than Haskell.
It doesn't have monads and the kind of abstractions that lets you modify control flow semantics. It doesn't even have the facilities to build abstract data types - which makes you work with less-abstract data, and realize that although some abstraction is useful, most of what is practiced today is useless.
APL / J / K promote, at the same time, container abstraction, and concrete down-to-the-metal real work.
I don't know how i feel about APL now, but I have some funny APL stories. One of my friends was the Jimi Hendrix of IBM APL2, he was amazing, unfortunately i can't tell the stories but when he learns haskell he'll be top notch. Another was the Dweezil Zappa of APL, he learned the language as a high school sophomore or junior from Iverson himself in Yorktown Heights.