Re: behaviour of assert on non-string errors

On Fri, Jan 17, 2014 at 6:43 AM, Justin Cormack <justin@specialbusservice.com> wrote:

If I have a function

function f() return nil, setmetatable({}, {__tostring = function()
return "error" end}) end

and I use assert on it I annoyingly get
assert(f())
stdin:1: bad argument #2 to 'assert' (string expected, got table)

Since 5.2, error() has been fixed to call __tostring it seems, so you
do not get "error object is not a string" messages.

I find non string error types to be incredibly useful, you can give
them predicates so the user can find out more about the error in the
code, but this means casual use of assert is hard. I end up redefining
assert always, just to add the tostring call.

Is there any reason for assert not to always call tostring?

Justin

Justin, I agree with you on this. There are several things that I don't like about assert and error, which leads me to wrap them up in my own function. Something like:

function assertx(cond, ...)

if cond then

return ...

else

return cond, tostring(select(1, ...)):format(select(2, ...))

end

----

I won't pretend to know the real answer, but I always assume it has to do with two things, in order:

1: Doing it the full-way (check the metatable, etc) adds complexity in functions that you want as fast as possible (assert fits that bill, error does not, as far as I can tell).

2: You can always wrap it and Lua is used in small / time critical systems, so the bar for 1 is sometimes rather low, given that there are a million ways to get around it (monkey patching, etc).

-Andrew