Update mpca_lang() example by add null terminated argument

[bruhtus]

First of all, thank you for the book. This book help me a lot when i am trying to learn C, especially looking at the mpc parser source code (even though i still have no idea for the majority of the code 😅).

In the current version of mpc parser, we can add NULL at the end of arguments mpca_lang() to indicate that it's the end of argument when trying to find parser and prevent segfault error.

This changes update the snippet for the web version and example that use mpca_lang() by adding NULL at the end of mpca_lang() argument.

This changes also resolves issue #147.

[bruhtus]

I've been thinking, should we add some explanation about NULL at the end of mpca_lang() arguments in chapter 5 (the earliest chapter that introduce mpca_lang())?

Something like "NULL at the end of mpca_lang argument is to indicate the end of the parser used, the one we initialized using mpc_new()". 🤔

[orangeduck]

Thanks a lot!

I think this was probably the original reason I didn't add it in the first place: it is difficult to explain the purpose of it without getting into the weeds: what is NULL? What are pointers? What are variable argument functions? Why don't they already know the number of arguments they are provided etc. etc.

[bruhtus]

Yeah, it's kind of difficult to explain, especially on the earlier chapter. But on the other hand, there might be people who mistype the parser name and get confused why they are getting segmentation fault.

My consideration of updating mpca_lang() example is to prevent some reader from giving up because they have no idea what causing the segmentation fault. But i see your point too, especially if we need to explain va_arg() and so on, the reader might get overwhelm and not continue reading instead 😅

If updating mpca_lang() example make the learning path become more convoluted, maybe we can add note "If there are some error with mpc, you can check mpc github repo" or something like that. The note i'm talking about is like this:

And we will leave it to the reader if they are curious enough to find out what causing the segmentation fault.

That's the solution that i can think of at the moment.