Fair enough. I suppose decades of review have made the Haskell source a bit cleaner.

It always makes me a bit disappointed to review the definition of the for macro in the Clojure source :(

AFAIK not nessecarily.
The way the clojure language bootstraps itself causes some core functions to be written in a weird reduced subset of the langauage.
Also a lot of the core functions are optimized in nonintuitive ways which might not be idiomatic either.

Besides idiomaticity, I don't think that reimplementing core functions is an easy (8 or 7 Kyu) task in clojure. Some of the design choises of clojure are weird to newcommers (see this super long discussion about the time complexity of last https://groups.google.com/forum/#!topic/clojure/apkNXk08Xes), and the devil is often in the detail when it comes to interactions between high level abstractions (seq) and lowlevel constructs (vectors and maps).

I don't know, I ported this from Haskell, where the "best practices" generally looks identical to the code in Prelude.hs.

Wouldn't the best practices for clojure be to write something like the original clojure.core/last source code?

Yeah but reimplementing core functionality can't be idiomatic by definiton. Because using the buildin would be the idiomatic way.