But at the face value, it's hard not to wonder whether you'd like to preserve a functional program, or its source code, or its architectural and design ideas?
Either way, your current perception of the program is likely tied to the current technology or perhaps even whole ecosystem around it.
So to preserve something like that, you'd need more than just storage.
If it's just for the source code, as in text, then as golden rule of backup goes - keep many copies in distributed but known locations. In other words - diversify and distribute. Whatever storage digital, or analog, or organic, as in human memory (storytelling is a type of storage too).
Though, likely, you mean the functional program. Thus you'd need to preserve the platform too, along with build tools. So, at least some system specs need to be preserved or a VM image for more or less stable virtualization envs.