by eru | 2025-08-14 04:16:24+00:00
> If you iterate by const reference over a const container, and you make every assign-once variable in the loop body const (or in Rust: just not mut), is there any advantage to tail recursion except someone on the internet said it's the proper functional style?

Function calls can express all kinds of useful and interesting control flow. They are so useful that even people who love imperative programming use functions in their language. (Early and primitive imperative programming languages like very early Fortran and underpowered dialects of BASIC didn't have user defined functions.)

So we established that you want functions in your language anyway. Well, and once you properly optimise function calls, what's known as tail call optimisation, you notice that you don't need no special purpose loops (nor goto) built into your language. You can define these constructs as syntactic sugar over function calls. Just like you can define other combinators like map or filter or tree traversals.

See how in the bad old days, Go had a handful of generic functions and data structures built-in (like arrays), but didn't allow users to define their own. But once you add the ability for users to define their own, you can remove the special case handling.

And that's also one thing C++ does well: as much as possible, it tries to put the user of the language on the same footing as the designers.

When 'map' or 'filter' are the best construct to express what you want to say, you should use them. When a 'for'-loop is the best construct, you should use it. (And that for-loop could be defined under the hood as syntactic sugar on top of function calls.) The scenario your concocted is exactly one where a foreach-loop shines.

Though to be a bit contrarian: depending on what your loop does, it might be useful to pick an ever more constrained tool. Eg if all you do run one action for each item, with no early return and you are not constructing a value, you can use something like Rust's 'foreach' (https://docs.rs/foreach/latest/foreach/). If you transform a container into another container (and no early return etc), you can use 'map'. Etc.

The idea is to show the reader as much as possible what to expect without forcing them to dive deep into the logic. The transformation in a 'map' might be very complicated, but you know the shape of the result immediately from just spying that it's a 'map'.

When you see the for-loop version of the above, you have to wade through the (complicated) body of the loop just to convince yourself that there's no early return and that we are producing a new container with exactly the same shape as the input container.

> I think functional programming contains some great ideas to keep state under control, but there is no reason to ape certain superficial aspects. E.g. the praise of currying in Haskell tutorials really grinds my gears, I think it's a "clever" but not insightful idea and it really weirds function signatures.

Yes, that's mixing up two separate things. Haskell doesn't really need currying. All you need for Haskell to work as a language is a convenient way to do partial application. So if Haskell (like OCaml) used tuples as the standard way to pass multiple arguments, and you had a syntactically convenient way to transform the function (a, b, c) -> d into (b, c) -> d by fixing the first argument that would get you virtually all of the benefits Haskell gets from pervasive currying without the weird function signatures.

In practice, people tend to get used to the weird function signatures pretty quickly, so there's not much pressure to change the system.