Re: assert with formatting

[Hisham <h@...>]

On 13 July 2018 at 11:23, Pierre Chapuis <catwell@archlinux.us> wrote:
>>   Um, but wouldn't
>>
>>   format_assert(a == 1 , "'a' should be '%d', but was '%s'",1,tostring(a))
>>
>> *also* compose the message even if a equals 1?  Lua doesn't do lazy
>> evaluation.
>
> It would call `tostring(a)` in all cases, but it would not call `string.format` because that branch of the `if` would never be evaluated.
>
> I am not entirely sure if there is a real performance gain, though.

Not only the cost of running string.format, but also memory:

for i = 1, N do
   local a = do_something(i)
   assert(a == 1, ("`a` should be `%d`, but was `%s`"):format(1,a))
end

This produces N strings in memory to be garbage collected (adding time
to GC runs), while this doesn't:

for i = 1, n do
   local a = do_something(i)
   format_assert(a == 1, "`a` should be `%d`, but was `%s`", 1, a)
end

(Note the removed tostring(), or else we'd still get N strings; so if
necessary the implementation of format_assert should do that instead.)

Plus, whether the above is true or not is also
implementation-dependent: Lua 5.3 stores short strings up to 40 bytes
in TValues, so in that version the above incurs or not in GC costs
depends on the length of the error message.

-- Hisham