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C4 – C in 4 functions

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Re: C4 – C in 4 functions

#81

Post by rui314 »

JoeAltmaier wrote: post #13
cbhl wrote: post #12

Earlier quoted context omitted.

Judging from the comments in c4.cpp, it probably only supports enough of a subset to compile itself. Granted, while building a parser that can parse (let alone compiling) the full C language is nontrivial, any undergrad should be able to build a parser and compiler for a sufficiently simple subset of it. (In my undergrad, we used this subset to build a "compiler" in second year: https://www.student.cs.uwaterloo.ca/~c…

You can build a C parser in an afternoon. It only has a few language constructs. Declarations are the hardest. Scanners are readily available for expressions and constants.

C is not a simple language as a CIL guy says [1]. I wrote my own C compiler [2] and I can say that writing a parser was harder than I thought. It would take more than half a day at least.

[1] http://www.cs.berkeley.edu/~necula/cil/cil016.html [2] https://github.com/rui314/8cc

Re: C4 – C in 4 functions

#82

Post by shangxiao »

abecedarius wrote: post #23

On a first skim, this looks really nice; complaints that it's unreadable are unfounded. The background that makes it readable are Wirth's Compiler Construction http://www.ethoberon.ethz.ch/WirthPubl/CBEAll.pdf plus precedence climbing http://en.wikipedia.org/wiki/Operator-precedence_parser#Prec...

It is unreadable. Lumping code together into big functions just so you can say "Look I've created a compiler in 4 functions" is pointless, unless your goal is to post it to HN to show everyone how clever you are.

This is not how you code when you work in a team or when you know some other poor soul has to come along and maintain it.

I suggest you take a look at [1] then go and read this excellent book by Martin Fowler: "Refactoring: Improving the Design of Existing Code" [2]

[1] https://en.wikipedia.org/wiki/Code_smell

[2] http://www.goodreads.com/book/show/44936.Refactoring

Re: C4 – C in 4 functions

#83

Post by zik »

rui314 wrote: post #81
JoeAltmaier wrote: post #13

Earlier quoted context omitted.

You can build a C parser in an afternoon. It only has a few language constructs. Declarations are the hardest. Scanners are readily available for expressions and constants.

C is not a simple language as a CIL guy says [1]. I wrote my own C compiler [2] and I can say that writing a parser was harder than I thought. It would take more than half a day at least. [1] http://www.cs.berkeley.edu/~necula/cil/cil016.html [2] https://github.com/rui314/8cc

Yes, I also wrote my own C interpreter and the parser was a lot more than a day's work.

Re: C4 – C in 4 functions

#84

Post by RubyPinch »

shangxiao wrote: post #82
abecedarius wrote: post #23

On a first skim, this looks really nice; complaints that it's unreadable are unfounded. The background that makes it readable are Wirth's Compiler Construction http://www.ethoberon.ethz.ch/WirthPubl/CBEAll.pdf plus precedence climbing http://en.wikipedia.org/wiki/Operator-precedence_parser#Prec...

It is unreadable. Lumping code together into big functions just so you can say "Look I've created a compiler in 4 functions" is pointless, unless your goal is to post it to HN to show everyone how clever you are. This is not how you code when you work in a team or when you know some other poor soul has to come along and maintain it. I suggest you take a look at [1] then go and read this excellent book by Martin Fowle…

I have a feeling that the project is a mix of for-fun and to-see-if-its-possible.

Not all programming is enterprise quality, some programming is intentionally not.

Being so dismissive of that, seems a little silly to me.

The demoscene doesn't exist because of a bunch of programmers trying to make the lives of every other programmer more difficult, it exists because people like the challenge.

Re: C4 – C in 4 functions

#85

Post by tdsamardzhiev »

aeonsky wrote: post #71
anigbrowl wrote: post #65

Earlier quoted context omitted.

I look to comments to tell me what a block of code is doing rather than why, eg 'Performs a Discrete Cosine Transform on the contents of the buffer' or 'Bubble sort algorithm rearranges the records in at least as much time as required to enjoy a nice cup of tea.' The 'why' of a very low-level tools like this is the sort of thing that needs to be explored at length in a paper or (in this case) a book, otherwise they'l…

I am a junior dev without a ton of experience so correct me if I'm wrong, but I strongly disagree. Comments should explain "why" something was written. Wouldn't the function name indicate what you are doing (and comments in the function)? This is especially true in business logic. reverseNaturalSortOrder(listOfItems); // case sensitive sort of items by reverse alphabetical order or reverseNaturalSortOrder(listOfItems…

Agreed - code should explain what code does. Duh.

Re: C4 – C in 4 functions

#86

Post by verytrivial »

cbhl wrote: post #12
TazeTSchnitzel wrote: post #8

Pretty damn impressive. Though I must wonder: how complete is it? What does it and does it not support? It's at least complete enough to be self-hosting, but beyond that? The code doesn't use that much of C.

Judging from the comments in c4.cpp, it probably only supports enough of a subset to compile itself. Granted, while building a parser that can parse (let alone compiling) the full C language is nontrivial, any undergrad should be able to build a parser and compiler for a sufficiently simple subset of it. (In my undergrad, we used this subset to build a "compiler" in second year: https://www.student.cs.uwaterloo.ca/~c…

Indeed. The lack of a case statement in the run loop is a bit of a giveaway. Excellent work nonetheless.

Re: C4 – C in 4 functions

#87

Post by userbinator »

shangxiao wrote: post #82
abecedarius wrote: post #23

On a first skim, this looks really nice; complaints that it's unreadable are unfounded. The background that makes it readable are Wirth's Compiler Construction http://www.ethoberon.ethz.ch/WirthPubl/CBEAll.pdf plus precedence climbing http://en.wikipedia.org/wiki/Operator-precedence_parser#Prec...

It is unreadable. Lumping code together into big functions just so you can say "Look I've created a compiler in 4 functions" is pointless, unless your goal is to post it to HN to show everyone how clever you are. This is not how you code when you work in a team or when you know some other poor soul has to come along and maintain it. I suggest you take a look at [1] then go and read this excellent book by Martin Fowle…

This also is not code that would need to be written by a team.

The functions may be big, but they also don't have all that much duplicate code inside them. The choice of 4 functions isn't arbitrary either - they nicely divide the problem into:

- next() - splits the source code into a series of tokens

- expr() - parses expressions

- stmt() - parses statements

- main() - starts the processing of the source, and also contains the main interpreter VM's execution loop

Code generation is integrated into the parsing, since it's generating code for a stack-based machine and that also very nicely follows the sequence of actions performed when parsing.

In fact I'm of the opinion that the obsession with breaking up code into tiny pieces (usually accompanied by the overuse of OOP) is harmful to the understanding of the program as a whole since it encourages looking at each piece independently and misses "seeing the forest for the trees".

In contrast, this is code that is designed to be easily read and understood by a single person, showing how very simple an entire compiler and interpreter/VM can be. It doesn't attempt to hide anything with thick layers upon layers of abstraction and deep chains of function calls, but instead is the "naked essence" of the solution to the problem.

Someone used to e.g. enterprise Java may find this style of code quite jarring to their senses, but that's only because they've grown accustomed to an environment in which everything is highly-abstracted and indirect, hiding the true nature of the solution. Personally, I think the simplicity and "nakedness" of this code has a great beauty to it --- it's a functional work of art.

Re: C4 – C in 4 functions

#88

Post by pvidler »

abecedarius wrote: post #23

On a first skim, this looks really nice; complaints that it's unreadable are unfounded. The background that makes it readable are Wirth's Compiler Construction http://www.ethoberon.ethz.ch/WirthPubl/CBEAll.pdf plus precedence climbing http://en.wikipedia.org/wiki/Operator-precedence_parser#Prec...

Most of it was readable, but the printf on lines 57--59 made me retch. I see what it's doing, but it's not what I'd call easily maintainable:

  printf("%8.4s", &"LEA ,IMM ,JMP ,JSR ,BZ ,BNZ ,ENT ,ADJ ,LEV ,LI ,LC ,SI ,SC ,PSH ,"
         "OR ,XOR ,AND ,EQ ,NE ,LT ,GT ,LE ,GE ,SHL ,SHR ,ADD ,SUB ,MUL ,DIV ,MOD ,"
         "OPEN,READ,CLOS,PRTF,MALC,MSET,MCMP,EXIT,"[*++le * 5]);

Re: C4 – C in 4 functions

#89

Post by Confusion »

shangxiao wrote: post #82
abecedarius wrote: post #23

On a first skim, this looks really nice; complaints that it's unreadable are unfounded. The background that makes it readable are Wirth's Compiler Construction http://www.ethoberon.ethz.ch/WirthPubl/CBEAll.pdf plus precedence climbing http://en.wikipedia.org/wiki/Operator-precedence_parser#Prec...

It is unreadable. Lumping code together into big functions just so you can say "Look I've created a compiler in 4 functions" is pointless, unless your goal is to post it to HN to show everyone how clever you are. This is not how you code when you work in a team or when you know some other poor soul has to come along and maintain it. I suggest you take a look at [1] then go and read this excellent book by Martin Fowle…

I suggest you consider how someone could know of, and understand, your suggestions very well, while still having the opinion he does. Don't underestimate other people.

Re: C4 – C in 4 functions

#90

Post by userbinator »

abecedarius wrote: post #76
jdp wrote: post #75

This is really similar to Marc Feeley's tinyc.c[0], which implements a C subset parser, code generator, and virtual machine in about ~300 lines of C. [0]: http://www.iro.umontreal.ca/~felipe/IFT2030-Automne2002/Comp...

That's a nice one. The big difference is that it doesn't compile itself -- not that there's anything magical about that, but it's a kind of threshold of seriousness.

One of the things I've wondered about is how small one could make an ISO C89-compliant compiler (that would be able to compile itself), and all these tiny compiler projects have inspired me to revisit that thought now... I've written pieces of compilers like expression parsers and tokenisers, and even then I felt like it wouldn't be so hard (if I had the time) to write a full compiler.

These are all great for dispelling the notion that all compilers somehow necessarily have to be greatly complex and impenetrable to anyone but highly-trained professionals and theoreticians. (Look at the Dragon Book, for example.)

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