On: Accidentally Turing-Complete

[tel]

This shows up all the time in the myriad Haskell eDSLs. Monad-based interfaces are obvious but throw away information that is useful for optimization (see http://homepages.inf.ed.ac.uk/slindley/papers/unembedding.pd... for how to get some of it back). Applicative interfaces are really amenable to optimization, though (see Facebook's Haxl project: http://www.reddit.com/r/haskell/comments/1le4y5/the_haxl_pro...)

[eru → tel]

Applicatives were the example that I had in mind. See also http://gergo.erdi.hu/blog/2012-12-01-static_analysis_with_ap..., or think about how regular expressions' limited power makes parsing with them a linear affair. (Incidentally, regular language parsers are Applicatives, too.)

[tel → eru]

Ah! That was the blog post I was looking for... but I kept searching for "Cactus Applicative" with no luck.