[AnimalMuppet]

> Loops and recursion can be translated into each other...

True.

> ... hence are of equal complexity in terms of reasoning.

False. That's like saying that Stokes' Theorem says that the integral of a differential form over the boundary of a manifold is equal to the integral of it's derivative over the whole manifold, and therefore the two integrals are equally easy to do. In fact, the two integrals are often not equally easy to do, which is the primary practical reason that we care about Stokes' Theorem - it gives us a way to convert hard integrals into easier ones.

You cannot use formal equivalence to prove practical ease of doing the problem.

But the situation isn't even that simple. A blanket statement that "loops are much easier to analyze and reason about" is also false. It depends on the situation, and often on who's doing the reasoning. Different people think in different ways.

[Jtsummers]

> You cannot use formal equivalence to prove practical ease of doing the problem.

Agree completely, but want to bring it back to another computing topic: Technically, anything that can be computed with one Turing-complete language can be computed with another. And every language clearly is not equivalent in terms of ease of understanding and reasoning about as every other language.

Trivially, compare computations in brainfuck to computations in C. Compare the recursive definition of tree traversal available in lisps to languages without recursion (some BASIC variants and others) where you have to jump through hoops and self-manage a stack.

[mafribe]

    compare computations in brainfuck to computations in C. 

I'd suggest that this is an orthogonal issue. It's better to compare two otherwise identical languages, one where Turing-completeness is achieved by loops, another where the same is done by recursion. (Or a language that offers both). Then formal reasoning is of the same complexity.

[mafribe]

    You cannot use formal equivalence to prove 

I'm afraid I have to disagree here. There is a mechanical translation from loops to recursion and back. Likewise, there is an associated mechanical translation from Hoare-triples and associated proofs for reasoning about loops to Hoare-triples and associated proofs for reasoning about recursion and back.

So in a hard way, the complexity of formal reasoning about the correctness of loops and recursion must be the same.

[AnimalMuppet]

That doesn't mean that the reasoning in a human brain is equally difficult, or equally error-prone. If nothing else, doing the transformations (and back) adds to the cognitive burden.

[mafribe]

That depends primarily on the programmer's experience, if you are used to loops but not recursion, then obviously the former is simpler than the latter, and vice versa. If you are equally familiar with both, they then thinking about it is equally difficult.