Where a spreadsheet file lives, and how to read and write it.

A file backend is two things that vary on their own: a format, which says how cells become bytes, and a store, which says where those bytes are and what it can promise about writing them. Sheetshow.Xlsx is the first format; Sheetshow.Store.Local is the first store.

workbook = Sheetshow.Workbook.xlsx("costs.xlsx")

A store is a value, like everything else here: the module that does the work, and whatever that module needs to find the file.

iex> store = Sheetshow.Store.local("costs.xlsx")
iex> {store.module, store.location}
{Sheetshow.Store.Local, "costs.xlsx"}

Versions, and what they are for

read/1 gives back the bytes and a version: whatever the store uses to tell one state of the file from another. write/3 takes that version back as a precondition, and a store that can refuse a write whose version no longer holds answers %Sheetshow.Error{reason: :conflict} instead of overwriting. That is :conditional_write, and writing a cell to a file replaces the whole file, so it matters more here than at Google; see What Sheetshow Can Promise. Not every store can promise it; Sheetshow.Backend.supports?/2 says which kind a workbook sits on.

A version is the store's own business, an entity tag here and a size and a modification time there, and nothing outside the store reads into it. Two atoms are reserved, so that a caller never has to guess what a missing one meant:

write/3 also takes :any, which writes over whatever is there.