Nested file:lines()

[Alexander Gladysh <agladysh@...>]

Hi, list!

OS X 10.5.8
Lua 5.1.4

Most likely this was discussed earlier (or even is documented
somewhere in the manual; or I'm doing something wrong :-) ).

I'm experiencing unexpected (by me) behaviour when nesting
file:lines() calls for the same file:

$ jot 3 >123.txt

$ cat >123.lua
local f = assert(io.open("123.txt"))
for l in f:lines() do
  print("O", l)
  for ll in f:lines() do -- Use the same file handle
    print("i", ll)
  end
end

$ lua 123.lua
O	1
i	2
i	3

Looks like lines iterator is shared for all :lines() calls. Not sure
if it is necessarily wrong, but perhaps it should be documented (am I
missed it?)

But what if I'd open two file handles for the same file?

$ cat >123-1.lua
local f = assert(io.open("123.txt"))
local ff = assert(io.open("123.txt"))
for l in f:lines() do
  print("O", l)
  for ll in ff:lines() do
    print("i", ll)
  end
end

$ lua 123-1.lua
O	1
i	1
i	2
i	3
O	2
O	3

Looks like subsequent calls for :lines() on the same handle are ignored.

Indeed: (IMO, this fact should be documented as well.)

$ cat >123-2.lua
local f = assert(io.open("123.txt"))

for l in f:lines() do
  print("O", l)
end

for l in f:lines() do
  print("i", ll)
end

$ lua 123-2.lua
O	1
O	2
O	3

So, the correct way is to re-open nested file handles on each:

$ cat >123-3.lua
local f = assert(io.open("123.txt"))
for l in f:lines() do
  print("O", l)
  local ff = assert(io.open("123.txt"))
  for ll in ff:lines() do
    print("i", ll)
  end
end

$ lua 123-3.lua
O	1
i	1
i	2
i	3
O	2
i	1
i	2
i	3
O	3
i	1
i	2
i	3

Alexander.