[BUG] TYPE command does not return a string in LUA

[manast]

Describe the bug
The TYPE command is supposed to return a string with the type of the key.

When called from a LUA script, the command does not return a string, it returns an empty table instead.

To reproduce

Just run this code and see the results:

127.0.0.1:6379> EVAL "redis.call(\"LPUSH\", \"dummy\", \"dummy\")\nlocal dummyType = redis.call(\"TYPE\", \"dummy\")\nredis.call(\"SET\", \"type\", type(dummyType))\nredis.call(\"SET\", \"len\", #dummyType)" 0
(nil)
127.0.0.1:6379> GET "type"
"table"
127.0.0.1:6379> GET "len"
"0"

Expected behavior

It should return a string, in this case the string "list"

Additional information

Works well if called directly on the redis-cli but not within a LUA script.

[enjoy-binbin]

This is indeed a very strange problem, and it look like it has always been there, i guess it is because TYPE did not return the "simple string" like GET

in this case, you can get the right result, by using dummyType['ok'] odd way

127.0.0.1:6379> EVAL "redis.call('LPUSH', 'dummy', 'dummy') local dummyType=redis.call('TYPE', 'dummy') redis.call('SET', 'type', dummyType['ok']) return dummyType" 0
list
127.0.0.1:6379> get type
"list"

let see what @MeirShpilraien would suggest, PTAL

[oranagra]

i'd like to call @itamarhaber to take a look too

[manast]

If the result is indeed in dummyType['ok'] and this issue has been there forever, maybe it is better to just document it instead of potentially breaking scripts?

I also found another inconsistent behavior, if you use redis.pcall on for example "ZADD", it returns an int if it succeeds and a table with 'err' if it fails. But this is not the same behavior as the example in the documentation where the command 'ECHO' is used, which always returns a table.

[MeirShpilraien]

As far as I understand, the idea was to distinguish between bulk string reply and simple string reply. I guess that bulk string reply are simply converted to Lua string because it is more common and simple string are returned as a Lua table with OK key. Maybe we can find a better way to distinguish them, but then we need to think about backward compatability.
I also believe this is already documented: https://redis.io/docs/manual/programmability/lua-api/#resp2-to-lua-type-conversion

[itamarhaber]

Indeed - simple strings are converted from RESP to Lua as such, and TYPE's return is also documented.

I agree this isn't ideal, but it has been this way for a long time, so changing the behavior now isn't recommended.

[oranagra]

@itamarhaber what do client libraries usually do with simple string return?
i understand that changing it is a breaking change, bug this feels a little awkward