Why does the Lua interactive interpreter stop letting me type things in?

Question

I am writing a program to find the standard deviation of a data set. I don't have a proper Lua editor so I am testing everything from the interactive interpreter.

In the code below, everything seems to work until I get to the diffsqrd function. After I call this function, the interpreter stops letting me type anything in. I have to close it and start over again. I have tested this function by itself, without the code before it and it works fine.

I tried saving the whole thing as a .lua file and running it with dofile but it did the same thing. I get nothing, and then I can no longer type into the interpreter. What is going on?

--a function to see if a file exists
function file_exists(file)
    local f=io.open(file, "r")
    if f then f:close() end
    return f ~= nil
end

--get all lines from a file, returns an empty 
--list/table if the file does not exist

function lines_from(file)
    if not file_exists(file) then return {} end
    lines = {}
    for line in io.lines(file) do 
        lines[#lines + 1] = line
    end
    return lines
end

--Put the .rec file into an array

y_positions=lines_from([[Z:\Octupole stuff\programming\y_p_test.rec]])


--functions to find the standard deviation of an array. 
--total, average, difference squared. I stop here because this is the     
--minimum code required to cause the problem. 


function total(a)
    local sum=0
        for i,v in ipairs(a) do sum = sum + v
    end
    return sum
end

function average(a)

        if #a==0 then mean=0
        else mean=total(a)/#a
    end
    return mean
end


function diffsqrd(a)
    local diff={}
    for i in ipairs(a) do 
        diff[i]=(a[i]-average(a))^2
    end
    return diff
end 

--Use the diffsqrd function on the .rec file. 

yd=diffsqrd(y_positions)
print(yd[1])