Replace for loops by functional approach

[blegat]

Using for loops for generators may create type instabilities, see https://discourse.julialang.org/t/multiplying-by-a-variable-in-a-constraint-causing-type-instability-in-code-warntype/32239 and forces us to create local scopes with let in the macro.
We could switch to a functional approach similarly to jump-dev/JuMP.jl#2070

Moreover, we currently we rewrite sum(... for i in ... but we don't rewrite sum(i -> ..., ...).

[odow]

There doesn't seem to be much appetite for this. I haven't seen any issues relating to it since 2019. Can we close?

[odow]

The new rewrite #170 adds support for sum(generator; init), but it doesn't add support for the two-argument sum(f, x). I think automatically rewriting that gets a little risky, because we don't know that:

sum(f, x; init) === sum(f(x) for a in x; init = init)

(It might depend on the types, although it'd be pretty poor form to change the semantics of sum like that.)

I also don't see how we would use functions instead of the loop, because in the functional case we wouldn't be able to rewrite in-place?

[odow]

Closing this as won't-fix. The loops are okay.

[blegat]

I also don't see how we would use functions instead of the loop, because in the functional case we wouldn't be able to rewrite in-place?

I don't see why but the functional approach might allocate because of closure so it might be best to stay with for-loops until closures are faster