gistfile1.rb

# Conditional decrement.
#
# Decrement the value of a key only if the current value is greater than
# a specified value.

require 'rubygems'
require 'redis'

r = Redis.new
cond_decr = <<LUA
    local value = tonumber(redis('get',KEYS[1]))
    if value == nil then return {err="Value at key is not integer"} end
    if value > tonumber(ARGV[1])
    then
        value = value - 1
        redis('set',KEYS[1],value)
    end
    return value
LUA

r.set(:x,4)
5.times {
    puts(r.eval(cond_decr,1,:x,0))
}

Why don't you declare global functions "set" and "get" instead of putting everything in a "redis" function? It would improve error message consistency: you wouldn't have to create a special error message to tell the user the command is unknown, he would get an "attempt to call nil" kind of error.

@antirez: Yeah, but:

1. That means that errors don't evaluate to false in an if statement.
2. It's very unlike normal Lua behavior, which would probably confuse people who already know Lua.

I still think that representing Redis errors with Lua errors would be the best solution. 90% of the time, errors in Redis represent exceptional conditions like improperly calling a command, which is exactly what Lua errors are used for. If there was a time where not directly erroring to the user was important, you could local ok, result = pcall(redis, "get", somekey). ok would contain true or false depending on whether it worked or not, and result would contain either the result of the operation or the error message.

The {ok = "OK"} is a pretty good idea though.

@doub: Because then he would have to define a C-level function for every single Redis command, not to mention polluting the global namespace to heck.

@leafstorm: If you have a single function, you have to do the dispatch inside the called C function anyway. If there are that many commands to bind, use a code generator. And to avoid polluting global namespace you can put it in a (preloaded) module.

@doub People could do something like https://gist.github.com/952677 if they need it. It might be better to leave such things to third party lua libs and keep the redis code simple.