> In the posters variant of a spreadsheet where a blank value is invalid, how would the common practice of SUM(column A) be handled, where column A has an unknown number of rows (that keeps on being added to)?
IME, that's not really all that important of a case, because this:
> Require all formulas that refer to column A be continually changed to reference the true amount of rows?
Is the normal way to handle it in Excel, so normal in fact that Excel has a couple of automated ways to handle it.
If you are doing a table (which is the only case where "sum of a column" really makes sense), then for Table T and column C, the formula is SUM(T[[#Data],[C]]).
The other is the fact that Excel automatically updates ranges in formulas as rows are inserted, etc. (though this doesn't work if you are appending, only if you are inserting within the range.)
The two cases I've seen where relying on blank row handling is a common way of dealing with data that can be added rather than relying on either structured references in tables or excels automated updates are:
1) Spreadsheets written prior to Excel supporting structured references in tables (or by people who learned Excel that long ago and haven't updated their skills), and
2) Creating forms with a fixed number of rows (usually, to be printed in hardcopy form, so that page layout is an issue), where simply zero-filling the relevant cells with a formula that doesn't display zero values would be a simple solution (since you have a fixed set of cells to fill.)
Of course, you could also have function fail by default on blank cells but take an optional parameter to treat blank cells as the appropriate identity, the same way Excel lookup functions rely on sorted data by default but can be given flag that tells them to find an exact match without relying on sorted input.
