Re: A small motivating example. Was: small incompatibility

[Rici Lake <lua@...>]

On 24-Oct-05, at 4:52 PM, Rici Lake wrote:
> In order to test this function, I had to implement the str.gsub 
> behaviour, of course. The rules I used are:
>
> 1) nil or false: leave the original string intact
> 2) true:         delete the match (i.e. replace with "")
> 3) string:       replace %x as in current implementation
> 4) table:        lookup the first capture in the table, and
>                  continue. (If another table is encountered,
>                  use the next capture as the index.)
> 5) function:     call the function with all captures. If it
>                  returns a boolean or nil, treat it as above;
>                  if it returns a string, use the string as the
>                  replacement without interpreting %x. Otherwise,
>                  throw an error.

I forgot to add the following comment:

There seems to me to be a semantic ambiguity about the definition of 
"number of matches" in the case where the match is rejected by the use 
of a nil/false value. On the one hand, there is a sense in which this 
is "not a match"; i.e., the semantic intent was to not do the replace. 
On the other hand, it's handy to be able to use gsub simply in order to 
count the number of matches:

function wc(input)
  local _, wordcount = input:gsub("%S+", nil)
  local _, linecount = input:gsub("\n", nil)
  return linecount, wordcount, #input
end

This will work with the patched version, and the copy-elimination 
optimization seems to help, but again, I haven't done any serious 
benchmarking. (This implementation of wc appears to be about 6 times 
slower than the standard C implementation, tested on a 2 MB file: 
0.207s vs 0.034s. With the nils changed to true, the time goes up to 
0.321s.)

> function wc(input)
>>   local _, wordcount = input:gsub("%S+", nil)
>>   local _, linecount = input:gsub("\n", nil)
>>   return linecount, wordcount, #input
>> end
> =wc(io.open"src/lstrlib.c":read"*a")
883     3272    23805
>
rlake@freeb:~/src/lua-5.1-alpha$ wc src/lstrlib.c
     883    3272   23805 src/lstrlib.c