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C4 – C in 4 functions

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Re: C4 – C in 4 functions

#101

Post by manish_gill »

pvidler wrote: post #88
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]);

I'd like to know why this was downvoted, and if people who down voted it can explain what the code is doing please?

Re: C4 – C in 4 functions

#102

Post by vesinisa »

Neat, but this is not a general purpose C interpreter. It seems to lack the preprocessor and is only able to execute the example program and itself because it has wrapper implementations for a static set of standard library functions including printf(), open(), read() and malloc().[1] Use a standard library function it does not readily support, and you're out of luck.

[1]: https://github.com/rswier/c4/blob/master/c4.c#L492-L498

Re: C4 – C in 4 functions

#103

Post by chris_wot »

boomlinde wrote: post #99
PhasmaFelis wrote: post #91

Earlier quoted context omitted.

> On a first skim, this looks really nice; complaints that it's unreadable are unfounded. Man, I can't even tell what this is supposed to be. My confusion is entirely founded. My thought process with articles like this goes something like "C in four functions, huh? Sounds like it could be clever. I'll just click and read the explanation... Oh, there isn't an explanation. Well, maybe this file will explain things! ...…

In the sense that some people won't have an idea of what's going on, this community altogether isn't particularly inclusive at all. Personally, I really don't want the topics this site covers to cater to a lowest common denominator, and I'm sure that isn't what you had in mind either, but that's the effect of taking "more of us" to mean more than you personally.

Actually, a lot of us probably don't understand what this is doing. I sure don't, but I really don't consider myself "lowest common denominator" either. I come here to learn, to be honest!

Re: C4 – C in 4 functions

#104

Post by amelius »

Next challenge: a fully compliant C++11 compiler written in 4 functions (1)

Then, I'd be impressed :)

(1) At maximum 500 lines per function, where each line is at most 200 characters long.

Re: C4 – C in 4 functions

#105

Post by JoeAltmaier »

vidarh wrote: post #94
sedachv wrote: post #79

Earlier quoted context omitted.

Exactly. You have to drop hooks into the lexer from the parser, and by the time you're done you end up with just as much code. Only it's slower than a recursive-descent parser would be, and a lot of the time parsing speed really matters because it shows up as user-visible latency.

While I have a strong dislike for lex/yacc and descendants, the "hooks" you need for C are trivial. As far as I remember the only thing you need is an ability for the lexer to check whether or not a given identifier is a variable or type. Even that is only needed if you want to report more specific information up to the parser. E.g. Clang doesn't. In Clang it is instead the parser that looks up the information in ord…

Right but by then the rule matching has been done - now you're writing code to distinguish rules instead of code to handle rules.

The advantage of a parser tool is, it makes the language constructs explicit in the code - here be assignment, here be for() etc. That has some value.

Re: C4 – C in 4 functions

#106

Post by sklogic »

dabockster wrote: post #51
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...

complaints that it's unreadable are unfounded Not exactly. You have to remember that language and compiler design require a LOT of work and experience to understand, and that many programmers will only see this as, frankly, spaghetti. I think it could have used some more block comments, but that's just me.

Basically, what you're saying is that any toy compiler example should be accompanied with a copy of the Dragon Book.

Re: C4 – C in 4 functions

#107

Post by maffydub »

manish_gill wrote: post #101
pvidler wrote: post #88

Earlier quoted context omitted.

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]);

I'd like to know why this was downvoted, and if people who down voted it can explain what the code is doing please?

It's taking an integer (representing an operation) and printing out the name of that operation.

First thing to say is that "* ++le" is the integer representing the operation to perform. This basically walks through the array of instructions returning each integer in turn.

Starting at the beginning of the line, we have "printf" with a format string of "%8.4s". This means print out the first 4 characters of the string that I pass next (padded to 8 characters). There then follows a string containing all of the operation names, in numerical order, padded to 4 characters and separated by commas (so the start of each is 5 apart). Finally, we do a lookup into this string (treating it as an array) at offset "* ++le * 5", i.e. the integer representing the operation multipled by 5 (5 being the number of characters between the start of each operation name). Doing this lookup gives us a char, but actually we wanted the pointer to this char (as we want printf to print out this char and the following 3 chars), so we take the address of this char (the & at the beginning of the whole expression).

It's concise, but not exactly self-documenting.

Does that make sense?

(I didn't downvote.)

Re: C4 – C in 4 functions

#108

Post by WhitneyLand »

sklogic wrote: post #106
dabockster wrote: post #51

Earlier quoted context omitted.

complaints that it's unreadable are unfounded Not exactly. You have to remember that language and compiler design require a LOT of work and experience to understand, and that many programmers will only see this as, frankly, spaghetti. I think it could have used some more block comments, but that's just me.

Basically, what you're saying is that any toy compiler example should be accompanied with a copy of the Dragon Book.

You seem to suggest that a background in compiler theory is somehow table stakes for commenting on HN. Since many here are not developers, and many developers don't have a CS degree, a few contextual comments seem appropriate.

Re: C4 – C in 4 functions

#109

Post by shangxiao »

userbinator wrote: post #87
shangxiao wrote: post #82

Earlier quoted context omitted.

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 co…

Breaking up code is more than just removing redundancy - it's about exactly what you have written: "Encouraging looking at each piece independently". That actually has advantages, the main being that it is easy to understand how each piece operates independently of the other pieces - simply cohesion & coupling which is applicable to any language, not just "Java". I don't believe in my experience that I miss "seeing the forest for the trees" - I encourage you to try it, perhaps by learning some functional programming.

There is one obsession that I am tired of is people posting "X awesome thing in 120 lines of JavaScript" or "Y in 4 functions". Just because the problem is reduced to small as possible metric, it doesn't make it good.

PS: Also you mention negatively connotated terms like "OOP", "Java", "thick layers of abstraction" and "deep chains of function calls" as if you've ascertained that I'm some enterprise developer that doesn't have any C experience and wouldn't know simplicity if it bit me in the ass.

Re: C4 – C in 4 functions

#110

Post by peterfirefly »

maffydub wrote: post #107
manish_gill wrote: post #101

Earlier quoted context omitted.

I'd like to know why this was downvoted, and if people who down voted it can explain what the code is doing please?

It's taking an integer (representing an operation) and printing out the name of that operation. First thing to say is that "* ++le" is the integer representing the operation to perform. This basically walks through the array of instructions returning each integer in turn. Starting at the beginning of the line, we have "printf" with a format string of "%8.4s". This means print out the first 4 characters of the string…

How is that not self-documenting if one knows C?
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