07: Side Effect Free Passing of Parameters

[dschwartznyc]

Feature Request

There is a semantic mismatch between Rune and Python regarding parameter handling. Rune expects functions to be side-effect free, effectively treating parameters as immutable or passed-by-value.

In contrast, Python uses pass-by-assignment. When mutable objects (like lists or classes) are passed into a generated Python function, any modification to those objects reflects in the caller's scope, violating Rune’s functional integrity.

The Java implementation solves this by:

1. Using "Holders" and "Builders."
2. Performing a .toBuilder() call on assignment (effectively cloning).
3. Returning a .build() result.

Current Failure Case:

In the following Rune snippet, the holder should remain unchanged in the calling scope. In the current Python generator, updated would simply be a reference to holder, and add would mutate the original object.

type IntHolder:
    items int (0..*)

func AddItemToHolder:
    inputs:
        holder IntHolder (1..1)
        value int (1..1)
    output:
        updated IntHolder (1..1)

    set updated: holder
    add updated -> items: value

Potential Solutions:

The Python Runtime and Generator must replicate "pass-by-value" behavior. There are two primary alternatives:

1. Eager Copying: Deep-copy all mutable inputs upon function entry.
2. Copy-on-Write (CoW): Use a wrapper that only clones the underlying data if a mutation (like add or set) is attempted.