Error handling (nil + string return)

[catwell]

There are many libraries in Lua that return something on success and nil + something on error. I am wondering about the best way to support that pattern.

One such example is http.request in LuaSocket. In teal-types it is typed as returning string, integer, {string:string}, string, but in practice it returns this or nil, string.

We could write string | nil, integer | string, {string:string}, string but what I think would be best is something like (string, integer, {string:string}, string) | (nil, string) (which does not exist)..

How do you deal with this in your Teal codebases?

Note: this question came from playing with Redbean which has functions defined like this in its documentation:

Fetch(url:str[, body:str|{method=value:str, body=value:str, headers=table,...,...}])
├─→ status:int, {header:str=value:str,...}, body:str
└─→ nil, error:str