How to implement a spreadsheet?

Cedric Shock cedricpublic@REDACTED
Tue Apr 13 03:22:38 CEST 2004


Short version:
When a bound variable is used in the definition of a new function via 
fun() is the value of the variable copied into the function definition, 
or is a reference used?

Is there a short way to import a function from another module into yours 
so you can use it without the module: prefix?

Long version:

I would like to implement something like a spreadsheet in erlang. The 
ultimate goal in a project like this is to provide the simplest, most 
general, possible interface to the user. I have decided that the best 
possible solution would be if the formulas allowable for the user were 
plain erlang code. This would allow a user to do amazing things like 
make a formula that generates a function and stores it in a cell, and at 
a later timeframe a formula that applies a stored function to other data 
or functions. In addition it would remove other possible burdens like 
writing a parser for things like addition and subtraction or making a 
generator for functions that will work in formulas.

So, the problem breaks down like this: We have some data and a context, 
and a formula from the user for the value of one cell of data in the 
next time frame. Somehow we need to slip the data and context into the 
user's formula without them seeing it. A nice formula would be something 

"sqrt(Data(1, 1)^2 + Data(1, 2)^2)"

Here's the idea for how to do this. Define a function which takes as its 
arguments the position in the spreadsheet, and returns the value at that 
location. Then assign that function to the variable Data bound inside 
the evaluator. Then we will evaluate the function Formula in that 
context. It would look something like this:

CalculateCell(Data, Context, Formula) ->
	DataFunction = fun(X, Y) -> LookupData(Data, Context, X, Y) end,
	Bindings = erl_eval:add_binding('Data', DataFunction, ...),
	eval(Formula, Bindings)

So here's the question. Erlang does not have destructive assignment, so 
a variable like Data (which is going to be huge) can be safely passed 
around all over the place almost entirely by reference; I assume Erlang 
takes full advantage of an optimization like this. However, I do not 
know if it will do the same thing in my definition of a function. Will 
the function definition be in terms of references to Data and Context 
that would only get de-referenced when serialized, or is the entirety of 
Data copied into the definition of the function?

Another issue is that it would be very desirable to have the math 
functions available when the formula is evaluated. What module does an 
evaluation happen in? I assume it happens in the module of the code that 
called it. It would be desirable for things like math:sin to be usable 
in a formula just through calling sin. Is there a shorter way to import 
them into your module than:

sin(Theta) ->

cos(Theta) ->


Thanks in advance for your time and advice,

Cedric A. Shock

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