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Ticket Hash: b5ceb24111d7ecfdcf5bafefe82b61c60a7354d0
Title: Can't use the iterator from db:*rows as a variable
Status: Closed Type: Code_Defect
Severity: Minor Priority: Zero
Subsystem: Resolution: Not_A_Bug
Last Modified: 2020-01-02 09:33:59
Version Found In: 0.9.5-1
User Comments:
anonymous added on 2019-09-15 15:30:11:

In Lua, an iterator is a function that returns new values on successive calls, and the function can be passed around as a variable. However, for some reason this doesn't work with the db:*rows functions of lsqlite3:

local sqlite3 = require("lsqlite3complete")
local db = sqlite3.open_memory()

local rows = db:nrows("select 11 as qwe") for row in rows do print(row.qwe) end

This produces the following error message:

lua: s.lua:5: bad argument #1 to 'for iterator' (:sqlite3:vm expected, got nil)
stack traceback:
	[C]: in for iterator 'for iterator'
	s.lua:6: in main chunk
	[C]: in ?

The iteration only works if a *rows function is called directly in a `for` statement.

The same happens with db:rows, db:nrows, and db:urows functions and corresponding stmt:*rows functions for prepared statements. It affects both lsqlite3complete and lsqlite3 with a dynamically loaded sqlite3 lib.

My use-case is that I use Fennel, a Lisp language on top of Lua, and I need to pick just the first row from the results. As a functional language, Fennel doesn't have early returns or `break` statements, but could easily call the iterator just once. More generally, iterators could be used with higher-order functions.

Versions are as follows:

LSQLite3 0.9.5-1, installed with Luarocks.
Lua 5.3.5 and 5.1.5 installed with Homebrew (haven't tested with other versions).
MacOSX 10.13.6.

For comparison, here's Lua code creating an iterator as a var:

function create_iterator(limit)
    local i = 0
    return function()
        i = i + 1
        if i <= limit then return i end

iterator = create_iterator(10) for x in iterator do print(x) end

anonymous (claiming to be the ticket author) added on 2020-01-02 09:33:59:

Never mind, apparently the iterator protocol is different from how it's described in ‘Programming in Lua’—at least in 5.1 and 5.3.