[WalterBright]

To expand on the other reply, some related things:

1. don't do console I/O in leaf functions. Instead, pass a parameter that's a "sink" for output, and let the caller decide what do with it. This helps a lot when converting a command line program to a gui program. It also makes it practical to unit test the function

2. don't allocate storage in a leaf function if the result is to be returned. Try to have storage allocated and free'd in the same function. It's a lot easier to keep track of it that way. Another use of sinks, output ranges, etc.

3. separate functions that do a read-only gathering of data, from functions that mutate the data

Give these a try. I bet you'll like the results!

[samatman]

I heartily agree with #2 if the language isn't Zig. Which actually supports your point: allocating in leaf functions is idiomatic in Zig, and it works out fine, because there's no allocation without an Allocator, and even if that's passed in implicitly as part of a struct argument, error{OutOfMemory} will be part of the function signature. So there's no losing track of what allocates and what doesn't.

This actually supports your broader point about always passing state to functions, and never accessing it implicitly. Although I don't know that I agree with extending that to constants, but maybe with another several decades of experience under my belt I might come to.

Zig also makes it easy for 'constants' to change based on build-specific parameters, so a different value for testing, or providing an override value in the build script. I've found that to eliminate any problems I've had in the past with global constants. Sometimes, of course, it turns out you want those values to be runtime configurable, but as refactorings go that's a relatively straightforward one.

[WalterBright]

> So there's no losing track of what allocates and what doesn't.

Having an allocator implicitly passed in with a struct argument is not quite what I meant. D once had allocators as member functions, but that wound up being deprecated because the allocation strategy is only rarely tied to the struct.

[samatman]

There are some meaningful differences between Zig and D in this specific area, specifically, D uses exceptions and has garbage collection as the default memory strategy. That will surely result in different approaches to the leaf-allocation question being better for the one than for the other.

[chipdart]

> Give these a try. I bet you'll like the results!

It sounds like too many words to refer ro plain old inversion of control and CQRS. They're both tried and true techniques.