Make a finky syntax checker

Question

You are to make a program that can check the syntax of programs of its same language. For example, if you do it in python, it checks python syntax. Your program will receive a program on standard input, and verify whether or not its syntax is correct. If it is correct, output just "true" on standard output. If not, output just "false" on standard output.

Your program must be incorrect in one instance though. If fed its own source code, it will just output "false" on standard output. This is code golf, so shortest program wins!

Note: Although this isn't technically a quine, you must follow the quine rules, meaning you can't access your source code through the file system or whatever.

Note: You can't claim a program that could not be run due to a syntax error solves this challenge, since it needs to be runnable.

Answer 1

Ruby 2.0, 65 ^{_{76 164}} characters

eval r="gets p;$<.pos=0;`ruby -c 2>&0`;p$?==0&&$_!='eval r=%p'%r"

This uses Ruby's built-in syntax checker (ruby -c) to check the input's syntax, which means the code won't be evaluated.

Basic usage example:

ruby syntax.rb <<< foo
true
ruby syntax.rb <<< "'"
false
ruby syntax.rb < synxtax.rb # assumes the file was saved without trailing newline
false

Explanation

This solution is (was) based on the standard Ruby quine:

q="q=%p;puts q%%q";puts q%q

%p is the format specifier for arg.inspect, which can be compared to uneval: when evaling the string returned by arg.inspect, you (usually) get the original value again. Thus, when formatting the q string with itself as argument, the %p inside the string will be replaced with the quoted string itself (i.e. we get something like "q=\"q=%p;puts q%%q\";puts q%q").

Generalizing this type of quine leads to something like the following:

prelude;q="prelude;q=%p;postlude";postlude

This approach has one huge drawback though (at least in code-golf): All code needs to be duplicated. Luckily, eval can be used to get around this:

eval r="some code;'eval r=%p'%r"

What happens here is that the code passed to eval is stored inside r before eval is called. As a result, the full source code of the eval statement can be obtained with 'eval r=%p'%r. If we do this inside the evald code and ensure that the top level of our consists only of the one eval statement, that expression actually gives us the full source code of our program, since any additional code passed to eval is already stored inside r.

Side note: This approach actually allows us to write a Ruby quine in 26 characters: eval r="puts'eval r=%p'%r"

Now, in this solution the additional code executed inside eval consists of four statements:

gets p

First, we read all input from STDIN and implicitly save it into $_.

$<.pos=0

Then, we rewind STDIN so the input is available again for the subprocess we start in the next step.

`ruby -c 2>&0`

This starts Ruby in its built-in syntax checking mode, reading the source code from stdin. If the syntax of the supplied script (filename or stdin) is ok, it prints Syntax OK to its stdout (which is captured by the parent process), but in case of a syntax error, a description of the error is printed to stderr - which would be visible, so we redirect that into nirvana (2>&0) instead.

p$?==0&&$_!='eval r=%p'%r

Afterwards, we check the subprocess's exit code $?, which is 0 if the syntax was ok. Lastly, the input we read earlier ($_) is compared against our own source code (which, as I described earlier, can be obtained with 'eval r=%p'%r).

Edit: Saved 14 characters thanks to @histocrat!

Answer 2

Rebol - 69 or 7475 fully compliant with all rules

New working versions thanks to @rgchris! Not sure if the first fails the "don't access the source" requirement as the interpreter holds the loaded and parsed code it has been passed as a cmd line parameter in the system object (system/options/do-arg) which is used to recognise itself.

probe not any[error? i: try[load/all input]i = system/options/do-arg]

This one follows all of the rules:

do b:[i: input prin block? if i <> join "do b:" mold b [try [load/all i]]]

Example usage:

First printing a valid integer, the second printing an invalid integer.

echo "rebol [] print 123" | rebol --do "probe not any[error? i: try[load/all input]i = system/options/do-arg]"
true
echo "rebol [] print 1w3" | rebol --do "probe not any[error? i: try[load/all input]i = system/options/do-arg]"
false
echo "probe not any[error? i: try[load/all input]i = system/options/do-arg]" | rebol --do "probe not any[error? i: try[load/all input]i = system/options/do-arg]"
false 

Fully compliant version:

echo 'rebol [] 123' |r3 --do 'do b:[i: input prin block? if i <> join "do b:" mold b [try [load/all i]]
true
echo 'rebol [] 123a' |r3 --do 'do b:[i: input prin block? if i <> join "do b:" mold b [try [load/all i]]]'
false
echo 'do b:[i: input prin block? if i <> join "do b:" mold b [try [load/all i]]]' |r3 --do 'do b:[i: input prin block? if i <> join "do b:" mold b [try [load/all i]]
false

Explanation:

First version

This uses Rebols built-in load function to parse and load the code from stdin but it does not execute it.

The try block catches any syntax errors and the error? function converts the error to a simple boolean.

The i = system/options/do-arg compares the input from stdin (assigned to i) with the code passed on the do-arg argument (sneaky but very golf :).

any is a great function which returns true if any-thing in the block evaluates to true (for example, any [ false false true ] would return true).

not then just inverts the boolean to give us the correct answer and probe displays the contents of the returned value.

Fully compliant version

Let's go through this in order ...

Assign the word b to the block [] that follows.

Use the do function to interpret the do dialect in the b block.

Inside the b block ...

Set the word i to refer to the contents of stdin (input).

Now, if we join the string "do b:" to the mold'ed block b and it is not equal (<>) to the stdin input i then we try to load the input i.

If the result is a block then we have load'ed the passed data correctly otherwise we would receive a none from the failed if.

Use prin to display the result of block? which returns true if the result is a block. Using prin as opposed to print does not display a carriage return after the output (and it saves us another char).

Answer 3

I think this follows the rules:

JS (✖╭╮✖)

function f(s){if(s==f.toString())return false;try{eval(s)}catch(e){return false}return true}

The code will be evaluated if it's correct.

Need to take a look at arrow notation to see if it can't be shortened more.

!function f(){try{s=prompt();return"!"+f+"()"!=s?eval(s):1}catch(e){return 0}}()

After a couple failed attempts and reverts - new version!

!function f(){try{s=prompt();"!"+f+"()"!=s?eval(s):o}catch(e){return 1}}()

And I'm back!

!function f(){try{s=prompt();"!"+f+"()"!=s?eval(s):o}catch(e){return !(e instanceof SyntaxError)}}()

And I'm gone! Unfortunately due to nature of eval and thanks to @scragar (damn you @scragar!) this approach will not work (seeing as throw new SyntaxError is valid JS code, which ticks this method of) - as such, I'd say it's impossible to create a syntax checker (at least using eval or any variation thereof)

(*see the comments!)

Answer 4

Javascript - 86 82

!function $(){b=(a=prompt())=='!'+$+'()';try{Function(a)}catch(e){b=1}alert(!b)}()

Paste into your browser's Javascript console to test.
Explanation:

// Declare a function and negate it to call it immediately
// <http://2ality.com/2012/09/javascript-quine.html>
!function $ () { 
  // Concatenating $ with strings accesses the function source
  invalid = (input = prompt()) == '!' + $ + '()';
  try {
    // Use the Function constructor to only catch syntax errors
    Function(input)
  }
  catch (e) {
    invalid = 1
  }
  alert(!invalid)
// Call function immediately
}()

Note: @m.buettner brought up the point that the program returns true for a naked return statement, such as return 0;. Because Javascript doesn't throw a syntax error for an illegal return statement until it actually runs (meaning code like if (0) { return 0; } doesn't throw a syntax error), I don't think there's any way to fix this short of writing a Javascript parser in Javascript. For example, consider the code:

while (1) {}
return 0;

If the code is executed, it will hang because of the loop. If the code is not executed, no error will be thrown for the illegal return statement. Therefore, this is as good as Javascript can get for this challenge. Feel free to disqualify Javascript if you feel that this doesn't fulfill the challenge sufficiently.

Answer 5

Haskell - 222 bytes

Note that this uses a true parser. It does not depend on eval like functions of dynamic languages.

import Language.Haskell.Parser
main=interact(\s->case parseModule s of ParseOk _->if take 89s=="import Language.Haskell.Parser\nmain=interact(\\s->case parseModule s of ParseOk _->if take"then"False"else"True";_->"False")

This solution is not particularly pretty but does work.

Answer 6

Python (95)

c=raw_input()
try:compile('"'if sum(map(ord,c))==7860 else c,'n','exec');print 1
except:print 0

Answer 7

PHP - 140

<?exec("echo ".escapeshellarg($argv[1])." | php -l",$A,$o);echo$o|(array_sum(array_map(ord,str_split($argv[1])))==77*150)?"false":"true";//Q

Comment necessary to keep the 'hash' (a shameless copy of s,ɐɔıʇǝɥʇuʎs). Using php -l / lint to check for errors.

$ php finky_syntax_check.php '<?php echo "good php code"; ?>' 
true
$ php finky_syntax_check.php '<?php echo bad--php.code.; ?>'
false
$ php finky_syntax_check.php '<?exec("echo ".escapeshellarg($argv[1])." | php -l",$A,$o);echo$o|(array_sum(array_map(ord,str_split($argv[1])))==77*150)?"false":"true";//Q'
false
$ php finky_syntax_check.php '<?exec("echo ".escapeshellarg($argv[1])." | php -l",$A,$o);echo$o|(array_sum(array_map(ord,str_split($argv[1])))==77*150)?"false":"true";//D'
true // note that the last character was changed

Answer 8

C 174

Explanation -Wall needed to produce system error while still being compilable. The syntax error is no return 0; To enter via stdin in Windows console type Ctrl-Z after pasting and press enter.

Golfed

char c[256];int i;int main(){FILE *f=fopen("a.c","w");while(fgets(c,256,stdin)!=NULL){fputs(c,f);}fclose(f);i=system("gcc a.c -o -Wall a.exe");printf("%s",i?"false":"true");}

Ungolfed:

#include <stdio.h>
#include <stdlib.h>
char c[256];int i;
int main()
{
FILE *f=fopen("a.c","w");
while(fgets(c,256,stdin)!=NULL)
{
fputs(c,f);
}
fclose(f);
i=system("gcc a.c -o -Wall a.exe");
printf("%s",i?"false":"true");
}

Answer 9

T-SQL - 110

Fairly simple, I've been wanting to try a challenge on here for a while, finally got round to doing it. This is not the fanciest code, but I had fun nontheless.

The 'golfed' version.

BEGIN TRY DECLARE @ VARCHAR(MAX)='SET NOEXEC ON'+'//CODE GOES HERE//'EXEC(@)PRINT'TRUE'END TRY BEGIN CATCH PRINT'FALSE'END CATCH

A better formatted version.

BEGIN TRY 
    DECLARE @SQL VARCHAR(MAX)='SET NOEXEC ON'+'//CODE GOES HERE//'
    EXEC(@SQL)
    PRINT'TRUE'
END TRY 

BEGIN CATCH 
    PRINT'FALSE'
END CATCH

It's fairly self explanatory, it uses SET NOEXEC on which makes it just parse the query instead of returning any results. the rest is mostly the try/catch that I use to determine what I need to print.

EDIT: I should have added that this will technically fail for itself. because it uses dynamic SQL any single quotes in the input have to be doubled ' -> ''