Saying Excel doesn't have any hidden state is stretching things a bit--your formulas don't show up until you are directly on them, so mistakes (formula results that got turned into static data, for example) are hidden until you put your cursor directly onto them.

"formula results that got turned into static data"

This hints at one of the sources of problems with Excel. One good approach to using Excel is to always be in one of two modes: changing the structure (equivalent to coding) and entering values (equivalent to using software). Of course, at the early stages of designing something you will be entering lots of dummy values (e.g. ARPU) to check that your formulae work.

However, many people don't distinguish between collections (rows, columns or other contiguous areas) of cells which _should_ contain values, and those which should contain formulae. I, like many, prefer to indicate input cells with a yellow or orange background, so that I know everything else is a formula.

You can do even better:

- If you're done designing a spreadsheet, and expect to use it for a production process, then mark the input cells as unlocked, and protect all the sheets. Then the end user won't be able to mess up the formulae.

- Always write formulae in a way that they can be copied across or down whilst pointing to the right places. This can be achieved through use of one or more $ signs to fix a reference. I've seen a fair number of spreadsheets where there are a large number of similar formulae, but they have been entered/adjusted manually. This is fine only if you never make mistakes, and if no one else needs to change the formulae later or verify they are correct.

My point is that "formula results that got turned into static data" can be avoided with just a few easy rules. However, most people aren't taught these rules :(

> Always write formulae in a way that they can be copied across or down whilst pointing to the right places. This can be achieved through use of one or more $ signs to fix a reference.

Except if you are using modern version of Excel, don't general use $ references for this, go one step further and use names (for fixed individual datapoints where you would use two $s) or named tables with named columns (for the most common use of single $ references.) This is more self-explanatory and less error-prone.

I never use it, but you can toggle between showing formulas and showing values with CTRL + ` (grave accent) (http://office.microsoft.com/en-001/excel-help/display-or-hid...)

> Excel is inherently observable since it doesn't have any hidden state

Except that it does -- even in terms of simple numerical values what you see is not necessarily the actual value, but instead the result of passing the value through a format string that can vary by cell.

Sadly, most people don't use this feature, and are content to show values using the default format, even if it results in values displayed as 12904819.23 (which might be better displayed as 12.9m or 12.90m).

> don't even have a notion of being run

They actually do have such a notion - in Excel you can set the calculations to Manual and then you hit F5 to refresh everything in one go. Useful when you deal with current date and time.

It's actually F9. F5 opens the Go To dialog box.