Re: Possible Lua enhancements 2: Piping

[Mark Hamburg <mark@...>]

On Dec 18, 2008, at 9:36 AM, Luiz Henrique de Figueiredo wrote:

> > >   pipe(producer(), filter1, filter2, ..., consumer)
> > That won't work if the producer can generate multiple values and we
> > need to pass those along.
>
> How about
> 	pipe(producer, filter1, filter2, ..., consumer) (args-to-producer)

That seems workable. It's a bit of a pain from a clarity standpoint to  
split the producer from its arguments, but at least it keeps the pipe  
presentation clear.

Order of evaluation remains a bit less than ideal, but most of the  
cases where that would matter are probably suspect anyway.

	for k, v in pipe( pairs, filter_key_type( "string" ) ) do
		-- process pairs with key type string
	end

Or in my original example:

	for obj in pipe( io.lines, chunk_lines( "----------" ),  
chunk_to_object )( "objectlist.dat" ) do
		-- process objects from file
	end

This also has the benefit of encouraging one to define:

	iterate_objects_from_file = pipe( io.lines,  
chunk_lines( "----------" ), chunk_to_object )

So, it is probably more in keeping with Lua's general style.

One could also extend .. to support pipe construction yielding:

	for obj in ( io.lines .. chunk_lines( "-----------" ) ..  
chunk_to_object )( "objectlist.dat" ) do
		-- process objects from file
	end

The difference between this and my original piping proposal is  
basically some extra parentheses and the separation of the arguments  
to feed the pipe from the head of the pipe.

And, of course, pipe is now essentially reverse function composition  
which might itself make a play for the .. operator.

Since my initial impetus was driven by LINQ, I should go back and look  
at some of the standard LINQ examples to see how they would look.

Mark

P.S. pipe is probably a lot easier to write in C rather than Lua since  
it needs to do a lot of manipulation of multiple values as it passes  
all of the results from one function to the next function. It would be  
nice to improve that situation, but that's a whole different topic.