Make your language unusable

Question

Try to write some code in your language and make it not satisfying our criteria of being a programming language any more.

A language satisfies our criteria (simplified version for this challenge) of being a programming language if:

• It can read user input representing tuples of positive integers in some way.
• It can output at least two different possible results depending on the input.
• It can take two positive integers and add them (and the result can affect the output).
• It can take a positive integer and decide whether it is a prime (and the result can affect the output).
• For the purpose of this challenge, any kind of output that isn't an allowed output method for a normal challenge is ignored. So it doesn't matter whether the program can also play a piece of music, or posting via HTTP, etc.
• Update: You can also choose one or some of the allowed output methods, and ignore all the others. But you must use the same definition everywhere in the following criteria. And if your program can disable more than one output methods — that worths more upvotes.

Examples like making it not able to output, or disabling all the loop constructs so it won't be able to do primality test and making sure the user cannot re-enable them.

You should leave a place for inserting new code. By default, it is at the end of your code. If we consider putting the source code in that place in your answer and running the full code as a complete program the interpreter of a new language, that language should not satisfy the criteria.

But the inserted code must be executed in such a way like a language satisfying the criteria:

• The inserted code must be grammatically the same as something (say it's a code block in the following criteria) that generally do satisfy the criteria, from the perspective of whoever wants to write a syntax highlighter. So it cannot be in a string, comment, etc.
• The inserted code must be actually executed, in a way it is supposed to satisfy the criteria. So it cannot be in an unused function or sizeof in C, you cannot just execute only a non-functional part in the code, and you cannot put it after an infinite loop, etc.
• You can't limit the number of possible grammatically correct programs generated this way. If there is already something like a length limit in the language you are using, it shouldn't satisfy the criteria even if this limit is removed.
• You can't modify or "use up" the content of input / output, but you can prevent them from being accessed.
• These criteria usually only applies to languages without explicit I/O:
  • Your code should redirect the user input (that contains informations of arbitrary length) to the inserted code, if a code block isn't usually able to get the user input directly / explicitly in the language you are using.
  • Your code should print the returned value of the inserted code, if a code block isn't usually able to output things directly / explicitly in the language you are using.
  • In case you print the returned value, and it is typed in the language you are using, the returned type should be able to have 2 different practically possible values. For example, you cannot use the type struct {} or struct {private:int x;} in C++.

This is popularity-contest. The highest voted valid answer (so nobody spotted an error or all errors are fixed) wins.

Clarifications

• You shouldn't modify the code in the text form, but can change the syntax before the code is interpreted or compiled.
• You can do other things while the code is running. But the reason that it doesn't satisfy the criteria should be within the inserted code itself. It can error because of the interference of another thread, but not just be killed by another thread.
• All the specs basically means it should be grammatically likely satisfying the criteria if all the built-ins were not changed but not actually do. It's fine if you find any non-grammatical workarounds, such as passing the parameters to the code block correctly, but make them not able to be used in some way.
• Again, the inserted code must be actually executed. Code after an infinite loop or crashing is considered "not actually executed", thus not valid. Those answers might be interesting, but there are already some other infinite loop or crashing questions on this site, and you may find a more appropriate one to answer. If not, consider asking a new question. Examples of those questions are:
  • Shortest infinite loop producing no output
  • Why isn't it ending?
  • Loop without 'looping'
  • Challenge: Write a piece of code that quits itself
  • Ridiculous Runtime Errors
  • Crash your favorite compiler

Leaderboard

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Answer 1

JavaScript Shell

This will make the language completely unusable.

clear(this);

Isn't it nice how JavaScript has such a nice function to destroy itself?

---

This is pretty simple, the clear function completely empty an object. this refers to the global object clearing out everything including constructors and functions.

---

Because this clears everything, doing anything, even defining a literal will throw an error, making the language completely useless: _{*REPL environment not required. Uses the SpiderMonkey engine (shell not browser), the original JS engine.}

Answer 2

Emmental

;#33!

I know this isn't code golf, but the right tool for the job, you know...

The user's code can be inserted after the !.

Emmental is an interesting esolang which is based on rewriting the interpreter. Every single symbol (including the built-in ones) can be redefined as an arbitrary Emmental program. The language relies on this feature so much that it doesn't provide any looping constructs. Instead, you define recursive commands which appear in their own definitions.

This redefinition happens via !, which reads a character from the stack, and then reads a string from the stack until it encounters a ;. The character is then redefined to mean the program represented by that string.

That means, we can disable Emmental's looping features by redefining ! itself as the empty program. While all other Emmental code still runs perfectly fine, and many of the criteria of a programming language are still fulfilled, it is impossible to redefine any more symbols. Without this feature (and therefore, without being able to loop), Emmental can no longer test whether a number is prime.

Answer 3

PHP

One can completely kill PHP by setting the memory limit to 1.

It will completely die.

Try this:

<?php
    ini_set('memory_limit',1);

    //code here

This shouldn't even throw any error, since there isn't enough memory for that.

You can read more about the memory_limit directive

---

If the previous one is invalid, one can use output buffers:

<?php
    ob_start();

    //code here

    ob_clear();

This completely removes any output. Since the output buffer is still open, some other things accidentally left after the code won't be shown as well.

---

Using @fschmengler's idea:

<?php
    define('OB_START_LEVEL', ob_get_level());
    ob_start(function(){return'';}, -1, PHP_OUTPUT_HANDLER_CLEANABLE);
    //or, for PHP <5.3:
    //ob_start(create_function('','return"";'), -1, PHP_OUTPUT_HANDLER_CLEANABLE);

    //code here

    while(ob_get_level() > OB_START_LEVEL) ob_clear();

This will avoid the problem of deleting the automatically started output buffer, used to catch the output to be compressed.

This also prevents that the output buffer is deleted or flushed (sent to the browser). To re-inforce that, an output handler is added, that always returns an empty string.
Running ob_end_flush(); echo "Hello, world!"; won't produce anything, but would send the output with a plain ob_start();.

Thanks to @LucasTrzesniewski for exposing this issue!

Answer 4

x86 machine code in real mode (=> almost any DOS program)

00000000  6a 00 07 b9 00 04 30 c0  31 ff f3 aa              |j.....0.1...|
0000000c

i.e.

push 0
pop es
mov cx,400h
xor al,al
xor di,di
rep stosb

I hope you weren't too attached to your interrupt table.

Answer 5

Java

import java.io.*;
import java.lang.reflect.*;
public class Test2 {
    public static void main(String[] args) throws Exception {
        args = new String[0];
        System.setOut(new PrintStream(new ByteArrayOutputStream()));
        System.setErr(new PrintStream(new ByteArrayOutputStream()));
        System.setIn(new ByteArrayInputStream(new byte[0]));

        Field modifiersField = Field.class.getDeclaredField("modifiers");
        modifiersField.setAccessible(true);

        Class<?> fdClass = java.io.FileDescriptor.class;
        Field outField = fdClass.getDeclaredField("out");
        outField.setAccessible(true);
        modifiersField.setInt(outField, outField.getModifiers() & ~Modifier.FINAL);
        outField.set(null, new FileDescriptor());
        Field errField = fdClass.getDeclaredField("err");
        errField.setAccessible(true);
        modifiersField.setInt(errField, errField.getModifiers() & ~Modifier.FINAL);
        errField.set(null, new FileDescriptor());
        Field inField = fdClass.getDeclaredField("in");
        inField.setAccessible(true);
        modifiersField.setInt(inField, inField.getModifiers() & ~Modifier.FINAL);
        inField.set(null, new FileDescriptor());

        System.setSecurityManager(new SecurityManager(){
            private boolean exitAllowed = false;
            public void checkPermission(java.security.Permission perm) {
                String name = perm.getName();
                if(name.equals("setIO")
                        || name.equals("setSecurityManager")
                        || name.equals("writeFileDescriptor")
                        || name.equals("readFileDescriptor")
                        || name.equals("suppressAccessChecks")
                        || (perm instanceof FilePermission
                            && name.startsWith("/proc/self/fd/"))){
                    throw new SecurityException("Nope");
                }
                if(name.startsWith("exitVM") && !exitAllowed){
                    exitAllowed = true;
                    System.exit(0);
                }
            }
            public void checkExec(String cmd){
                throw new SecurityException("nope");
            }
        });

        // program here
    }
}

Edit: Counter-countermeasures are making this giant :(

Redirects stdin and stdout to null streams and replaces args with an empty array. Also uses enormous amounts of reflection hacks to make sure the standard IO is truly hidden. Finally, it sets a security manager to make sure the standard IO can't be recreated and that makes sure programs can't set the exit code.

Answer 6

Lua

_ENV=""

In Lua, _ENV is the environment that all global variables, functions, tables, etc are stored in. Defining it to just be an empty string means you can't define anything new, and all functions and variables are wiped. This means you can not output anything, take in input, or pretty much do anything.

Answer 7

Shakespeare Programming Language

Have Fun Mr Parser.

Romeo, a young man of Verona.
Juliet, a young lady of Verona.
Hamlet, another character which causes the crash.

Act I
Scene I The crash.

[Enter Romeo]
[Enter Juliet]
[Exit Hamlet]

In SPL, the built in parser which is downloaded with the program follows very specific rules as to what can happen in the script. One of such rules is that only two characters can be on stage at once. Also, making a character exit the stage who was never on stage will confuse it. The same goes for adding a character to the stage who is already on stage. When the parser receives an error, it will refuse to do ANYTHING else; you literally have to completely shutdown the program and the parser and then start up everything again.

P.S. If you have no idea how this language works, Google it. It's awesome.

Answer 8

Smalltalk

I'm not sure if this qualifies:

Smalltalk := Nil.

This deletes the entire run-time environment, hanging the object engine. The only way to fix this is to forcibly terminate the process and restart from backup.

For those that don't know, the way [Visual Works] Smalltalk works is slightly weird. It's like a mini-OS. When you start Smalltalk, you load a "memory image" into RAM, and it continues executing from where it left off. The entire Smalltalk IDE is written in Smalltalk and is dynamically modifiable.

In particular, Smalltalk is a dictionary containing all global variables. Most particularly, every time you declare a new class, a global variable with that name is created, pointing to the Class object for your new class. So setting Smalltalk to Nil (basically null) deletes all classes in the entire system. Even the GUI event handlers go poof.

I have no idea why this variable is even writable. Probably because it's a global variable, and therefore exists as an entry inside itself. (Does your head hurt yet? Did I mention that every object has a class, and classes are objects, so every class has a class? The class of a class is called a metaclass, but a metaclass is also an object, which therefore has a class...)

You could probably achieve a similar effect by clearing the dictionary rather than replacing it with null. Indeed, there's any number of things you could code to delete all the classes in the system, leaving you unable to do anything. But since the actual Smalltalk compiler is also a class... anything that breaks the language also kinda breaks the entire IDE, so...

Answer 9

PostScript

Yes, PostScript is a programming language. Moreover, it's a programming language where all language constructs are system-defined functions, which can be redefined...

/Magic 1000 dict def
systemdict {pop Magic exch {} put} forall
Magic begin

In English:

• Create an empty 1,000-element dictionary and name it Magic.
• For every key in systemdict, add the same key to Magic, with an empty definition ("{}").
• Push Magic onto the top of the dictionary stack.

From this moment on, every PostScript language command is defined to do nothing. AFAIK, it is impossible to escape from this condition.

(Technically, you're not "destroying" the old definitions, you're just shadowing them. If you could still execute end, that would pop Magic off the dictionary stack, un-shadowing all the commands and giving you your life back. But since end itself is also shadowed... it will do nothing now.)

Note that all commands will still execute... it's just that now they are defined to do nothing. You won't get any kind of error, it's just that nothing will happen. (Well, I suppose stack overflow will happen eventually...)

Answer 10

Haskell

There are a couple of possibilities here.

Boring idea #1: Define main to do nothing. Now no matter what other code you write, it can never execute. (Unless you manually run it from the REPL.)

Boring idea #2: Define a module with no public exports. Now no matter what other code your write, it can never execute.

Interesting idea: Disable all imports.

module Fubar where
import Prelude ()
foo = foo
-- More code here.

Now you can define functions which are visible and can be run... but they can't do anything. All standard Haskell types and functions are now hidden. (With the exception of a few things really deeply hard-wired into the language.)

Most particularly, you cannot perform any I/O whatsoever. You also cannot do machine-precision arithmetic. (Since Int, Double, etc are now undefined.)

You can still write lambda-calculus functions that do perform some real computation though. You just can't get any data into or out of the thing. But you could of course write another, separate module that calls the Fubar module above and does I/O on its behalf (thus proving that the code does execute and does do stuff).

Some subtleties:

• The dummy foo = foo declaration is needed to prevent anybody adding additional imports. (Imports cannot appear after declarations.)

• There are various non-standard Haskell language extensions that would enable you to climb out of this situation. But language extensions have to be switched on with a compiler pragma at the top of the file. (Or with a command-line switch to the compiler. I can't really prevent that one!)

Answer 11

Any program executing under Linux/x86(-64)

This program is written in C, but it can disrupt execution of any program running under Linux/x86 (-32 or -64). You prepend it to the command-line invocation of the program you want to disrupt.

It uses the debugger API to prevent the target program from producing any output. Specifically, all of the system calls that can communicate something to the world outside the process (most obviously write, of course, but also open when creating a file, the bulk of the socket API, kill when applied to another process, ...) will fail as if they were unimplemented. _exit is allowed, but the exit code is overwritten with a zero.

Unlike the previous edition of this answer, many programs can run nearly to completion under these conditions; it's just that all their work is wasted. For instance, if you do ./no-syscalls /bin/ls (assuming GNU coreutils ls) it reads the whole directory and formats it, and then all the write calls to produce output fail. (Anything that needs to open a bidirectional communication channel, though, such as all X11 clients, will fail at that point. I thought about allowing socket but not send, but it seemed too likely to open loopholes.)

There are several command-line options to tweak the behavior;

-a  log allowed system calls
-d  log denied system calls
-e  deny everything, not just output
-S  permit writes to stderr

Dynamically linked programs will not even get out of the dynamic linker in -e mode. -S obviously opens a huge hole in the policy, but it can be entertaining to watch programs moan about nothing working, e.g.

$ ./no-syscalls -daeS /bin/ls
syscall 59...
syscall 59 = 0
syscall 12 (denied) = -38
syscall 21 (denied) = -38
syscall 9 (denied) = -38
syscall 20...
/bin/ls: error while loading shared libraries: cannot create cache for search path: Cannot allocate memory
syscall 20 = 107
syscall 231...
Program exited, status = 0

You have to read log output with /usr/include/asm*/unistd.h open in another window, because this is already quite long enough.

Sadly, the debugger interfaces that this uses are only weakly consistent across Unix implementations, and are intrinsically CPU-specific. It would be relatively straightforward to port it to other CPU architectures (just add appropriate definitions of SYSCALL_*_REG), and it's probably possible to port it to any Unix that has ptrace, but you might need to muck with the syscall whitelist extensively as well as dealing with divergences in ptrace.

#define _GNU_SOURCE 1
#include <stddef.h>
#include <sys/types.h>
#include <sys/mman.h>
#include <sys/ptrace.h>
#include <sys/stat.h>
#include <sys/syscall.h>
#include <sys/user.h>
#include <sys/wait.h>
#include <errno.h>
#include <fcntl.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

#if defined __linux__
# define SYS_unimplemented -1L
# if defined __i386__
#  define SYSCALL_NUMBER_REG regs.orig_eax
#  define SYSCALL_ARG1_REG   regs.ebx
#  define SYSCALL_ARG2_REG   regs.ecx
#  define SYSCALL_ARG3_REG   regs.edx
#  define SYSCALL_ARG4_REG   regs.esi
#  define SYSCALL_RESULT_REG regs.eax
# elif defined __x86_64__
#  define SYSCALL_NUMBER_REG regs.orig_rax
#  define SYSCALL_ARG1_REG   regs.rdi
#  define SYSCALL_ARG2_REG   regs.rsi
#  define SYSCALL_ARG3_REG   regs.rdx
#  define SYSCALL_ARG4_REG   regs.r10
#  define SYSCALL_RESULT_REG regs.rax
# else
#  error "Need to know system call convention for this CPU"
# endif
#else
# error "Need to know system call convention for this OS"
#endif

static long
xptrace(int request, pid_t pid, void *addr, void *data)
{
  errno = 0;
  long rv = ptrace(request, pid, addr, data);
  if (rv == -1 && errno) {
    perror("ptrace");
    if (pid != 0) kill(pid, SIGKILL);
    exit(1);
  }
  return rv;
}
#define GET_REG_(pid, x) \
  xptrace(PTRACE_PEEKUSER, pid, (void*)offsetof(struct user, x), 0)
#define GET_REG(pid, x) GET_REG_(pid, SYSCALL_##x##_REG)
#define SET_REG_(pid, x, v) \
  xptrace(PTRACE_POKEUSER, pid, (void*)offsetof(struct user, x), (void*)v)
#define SET_REG(pid, x, v) SET_REG_(pid, SYSCALL_##x##_REG, v)

/* This function defines the system-call policy.  */
static int
deny_syscall(pid_t pid, int scnum, int deny_all, int allow_stderr)
{
  switch (scnum) {
  /* These syscalls are unconditionally allowed (when not in -e mode);
     they perform input, or change only process-local state. */
#ifdef SYS_access
  case SYS_access:
#endif
#ifdef SYS_alarm
  case SYS_alarm:
#endif
#ifdef SYS_arch_prctl
  case SYS_arch_prctl:
#endif
#ifdef SYS_brk
  case SYS_brk:
#endif
#ifdef SYS_capget
  case SYS_capget:
#endif
#ifdef SYS_clock_getres
  case SYS_clock_getres:
#endif
#ifdef SYS_clock_gettime
  case SYS_clock_gettime:
#endif
#ifdef SYS_clock_nanosleep
  case SYS_clock_nanosleep:
#endif
#ifdef SYS_close
  case SYS_close:
#endif
#ifdef SYS_dup
  case SYS_dup:
#endif
#ifdef SYS_dup2
  case SYS_dup2:
#endif
#ifdef SYS_dup3
  case SYS_dup3:
#endif
#ifdef SYS_epoll_create
  case SYS_epoll_create:
#endif
#ifdef SYS_epoll_create1
  case SYS_epoll_create1:
#endif
#ifdef SYS_epoll_ctl
  case SYS_epoll_ctl:
#endif
#ifdef SYS_epoll_ctl_old
  case SYS_epoll_ctl_old:
#endif
#ifdef SYS_epoll_pwait
  case SYS_epoll_pwait:
#endif
#ifdef SYS_epoll_wait
  case SYS_epoll_wait:
#endif
#ifdef SYS_epoll_wait_old
  case SYS_epoll_wait_old:
#endif
#ifdef SYS_eventfd
  case SYS_eventfd:
#endif
#ifdef SYS_eventfd2
  case SYS_eventfd2:
#endif
#ifdef SYS_faccessat
  case SYS_faccessat:
#endif
#ifdef SYS_fadvise64
  case SYS_fadvise64:
#endif
#ifdef SYS_fadvise64_64
  case SYS_fadvise64_64:
#endif
#ifdef SYS_fanotify_init
  case SYS_fanotify_init:
#endif
#ifdef SYS_fanotify_mark
  case SYS_fanotify_mark:
#endif
#ifdef SYS_fgetxattr
  case SYS_fgetxattr:
#endif
#ifdef SYS_flistxattr
  case SYS_flistxattr:
#endif
#ifdef SYS_fstat
  case SYS_fstat:
#endif
#ifdef SYS_fstat64
  case SYS_fstat64:
#endif
#ifdef SYS_fstatat64
  case SYS_fstatat64:
#endif
#ifdef SYS_fstatfs
  case SYS_fstatfs:
#endif
#ifdef SYS_fstatfs64
  case SYS_fstatfs64:
#endif
#ifdef SYS_ftime
  case SYS_ftime:
#endif
#ifdef SYS_futex
  case SYS_futex:
#endif
#ifdef SYS_getcpu
  case SYS_getcpu:
#endif
#ifdef SYS_getcwd
  case SYS_getcwd:
#endif
#ifdef SYS_getdents
  case SYS_getdents:
#endif
#ifdef SYS_getdents64
  case SYS_getdents64:
#endif
#ifdef SYS_getegid
  case SYS_getegid:
#endif
#ifdef SYS_getegid32
  case SYS_getegid32:
#endif
#ifdef SYS_geteuid
  case SYS_geteuid:
#endif
#ifdef SYS_geteuid32
  case SYS_geteuid32:
#endif
#ifdef SYS_getgid
  case SYS_getgid:
#endif
#ifdef SYS_getgid32
  case SYS_getgid32:
#endif
#ifdef SYS_getgroups
  case SYS_getgroups:
#endif
#ifdef SYS_getgroups32
  case SYS_getgroups32:
#endif
#ifdef SYS_getitimer
  case SYS_getitimer:
#endif
#ifdef SYS_get_kernel_syms
  case SYS_get_kernel_syms:
#endif
#ifdef SYS_get_mempolicy
  case SYS_get_mempolicy:
#endif
#ifdef SYS_getpeername
  case SYS_getpeername:
#endif
#ifdef SYS_getpgid
  case SYS_getpgid:
#endif
#ifdef SYS_getpgrp
  case SYS_getpgrp:
#endif
#ifdef SYS_getpid
  case SYS_getpid:
#endif
#ifdef SYS_getpmsg
  case SYS_getpmsg:
#endif
#ifdef SYS_getppid
  case SYS_getppid:
#endif
#ifdef SYS_getpriority
  case SYS_getpriority:
#endif
#ifdef SYS_getrandom
  case SYS_getrandom:
#endif
#ifdef SYS_getresgid
  case SYS_getresgid:
#endif
#ifdef SYS_getresgid32
  case SYS_getresgid32:
#endif
#ifdef SYS_getresuid
  case SYS_getresuid:
#endif
#ifdef SYS_getresuid32
  case SYS_getresuid32:
#endif
#ifdef SYS_getrlimit
  case SYS_getrlimit:
#endif
#ifdef SYS_get_robust_list
  case SYS_get_robust_list:
#endif
#ifdef SYS_getrusage
  case SYS_getrusage:
#endif
#ifdef SYS_getsid
  case SYS_getsid:
#endif
#ifdef SYS_getsockname
  case SYS_getsockname:
#endif
#ifdef SYS_getsockopt
  case SYS_getsockopt:
#endif
#ifdef SYS_get_thread_area
  case SYS_get_thread_area:
#endif
#ifdef SYS_gettid
  case SYS_gettid:
#endif
#ifdef SYS_gettimeofday
  case SYS_gettimeofday:
#endif
#ifdef SYS_getuid
  case SYS_getuid:
#endif
#ifdef SYS_getuid32
  case SYS_getuid32:
#endif
#ifdef SYS_getxattr
  case SYS_getxattr:
#endif
#ifdef SYS_inotify_add_watch
  case SYS_inotify_add_watch:
#endif
#ifdef SYS_inotify_init
  case SYS_inotify_init:
#endif
#ifdef SYS_inotify_init1
  case SYS_inotify_init1:
#endif
#ifdef SYS_inotify_rm_watch
  case SYS_inotify_rm_watch:
#endif
#ifdef SYS_ioprio_get
  case SYS_ioprio_get:
#endif
#ifdef SYS_kcmp
  case SYS_kcmp:
#endif
#ifdef SYS_lgetxattr
  case SYS_lgetxattr:
#endif
#ifdef SYS_listxattr
  case SYS_listxattr:
#endif
#ifdef SYS_llistxattr
  case SYS_llistxattr:
#endif
#ifdef SYS_lookup_dcookie
  case SYS_lookup_dcookie:
#endif
#ifdef SYS_lseek
  case SYS_lseek:
#endif
#ifdef SYS_lstat
  case SYS_lstat:
#endif
#ifdef SYS_lstat64
  case SYS_lstat64:
#endif
#ifdef SYS_madvise
  case SYS_madvise:
#endif
#ifdef SYS_mbind
  case SYS_mbind:
#endif
#ifdef SYS_mincore
  case SYS_mincore:
#endif
#ifdef SYS_mlock
  case SYS_mlock:
#endif
#ifdef SYS_mlockall
  case SYS_mlockall:
#endif
#ifdef SYS_mprotect
  case SYS_mprotect:
#endif
#ifdef SYS_mremap
  case SYS_mremap:
#endif
#ifdef SYS_munlock
  case SYS_munlock:
#endif
#ifdef SYS_munlockall
  case SYS_munlockall:
#endif
#ifdef SYS_munmap
  case SYS_munmap:
#endif
#ifdef SYS_name_to_handle_at
  case SYS_name_to_handle_at:
#endif
#ifdef SYS_nanosleep
  case SYS_nanosleep:
#endif
#ifdef SYS_newfstatat
  case SYS_newfstatat:
#endif
#ifdef SYS_nice
  case SYS_nice:
#endif
#ifdef SYS_oldfstat
  case SYS_oldfstat:
#endif
#ifdef SYS_oldlstat
  case SYS_oldlstat:
#endif
#ifdef SYS_oldolduname
  case SYS_oldolduname:
#endif
#ifdef SYS_oldstat
  case SYS_oldstat:
#endif
#ifdef SYS_olduname
  case SYS_olduname:
#endif
#ifdef SYS_pause
  case SYS_pause:
#endif
#ifdef SYS_perf_event_open
  case SYS_perf_event_open:
#endif
#ifdef SYS_personality
  case SYS_personality:
#endif
#ifdef SYS_pivot_root
  case SYS_pivot_root:
#endif
#ifdef SYS_poll
  case SYS_poll:
#endif
#ifdef SYS_ppoll
  case SYS_ppoll:
#endif
#ifdef SYS_prctl
  case SYS_prctl:
#endif
#ifdef SYS_pread64
  case SYS_pread64:
#endif
#ifdef SYS_preadv
  case SYS_preadv:
#endif
#ifdef SYS_prlimit64
  case SYS_prlimit64:
#endif
#ifdef SYS_pselect6
  case SYS_pselect6:
#endif
#ifdef SYS_query_module
  case SYS_query_module:
#endif
#ifdef SYS_read
  case SYS_read:
#endif
#ifdef SYS_readahead
  case SYS_readahead:
#endif
#ifdef SYS_readdir
  case SYS_readdir:
#endif
#ifdef SYS_readlink
  case SYS_readlink:
#endif
#ifdef SYS_readlinkat
  case SYS_readlinkat:
#endif
#ifdef SYS_readv
  case SYS_readv:
#endif
#ifdef SYS_recvfrom
  case SYS_recvfrom:
#endif
#ifdef SYS_recvmmsg
  case SYS_recvmmsg:
#endif
#ifdef SYS_recvmsg
  case SYS_recvmsg:
#endif
#ifdef SYS_remap_file_pages
  case SYS_remap_file_pages:
#endif
#ifdef SYS_request_key
  case SYS_request_key:
#endif
#ifdef SYS_restart_syscall
  case SYS_restart_syscall:
#endif
#ifdef SYS_rt_sigaction
  case SYS_rt_sigaction:
#endif
#ifdef SYS_rt_sigpending
  case SYS_rt_sigpending:
#endif
#ifdef SYS_rt_sigprocmask
  case SYS_rt_sigprocmask:
#endif
#ifdef SYS_rt_sigreturn
  case SYS_rt_sigreturn:
#endif
#ifdef SYS_rt_sigsuspend
  case SYS_rt_sigsuspend:
#endif
#ifdef SYS_rt_sigtimedwait
  case SYS_rt_sigtimedwait:
#endif
#ifdef SYS_sched_getaffinity
  case SYS_sched_getaffinity:
#endif
#ifdef SYS_sched_getattr
  case SYS_sched_getattr:
#endif
#ifdef SYS_sched_getparam
  case SYS_sched_getparam:
#endif
#ifdef SYS_sched_get_priority_max
  case SYS_sched_get_priority_max:
#endif
#ifdef SYS_sched_get_priority_min
  case SYS_sched_get_priority_min:
#endif
#ifdef SYS_sched_getscheduler
  case SYS_sched_getscheduler:
#endif
#ifdef SYS_sched_rr_get_interval
  case SYS_sched_rr_get_interval:
#endif
#ifdef SYS_sched_setaffinity
  case SYS_sched_setaffinity:
#endif
#ifdef SYS_sched_setattr
  case SYS_sched_setattr:
#endif
#ifdef SYS_sched_setparam
  case SYS_sched_setparam:
#endif
#ifdef SYS_sched_setscheduler
  case SYS_sched_setscheduler:
#endif
#ifdef SYS_sched_yield
  case SYS_sched_yield:
#endif
#ifdef SYS_select
  case SYS_select:
#endif
#ifdef SYS_setfsgid
  case SYS_setfsgid:
#endif
#ifdef SYS_setfsgid32
  case SYS_setfsgid32:
#endif
#ifdef SYS_setfsuid
  case SYS_setfsuid:
#endif
#ifdef SYS_setfsuid32
  case SYS_setfsuid32:
#endif
#ifdef SYS_setgid
  case SYS_setgid:
#endif
#ifdef SYS_setgid32
  case SYS_setgid32:
#endif
#ifdef SYS_setgroups
  case SYS_setgroups:
#endif
#ifdef SYS_setgroups32
  case SYS_setgroups32:
#endif
#ifdef SYS_setitimer
  case SYS_setitimer:
#endif
#ifdef SYS_setns
  case SYS_setns:
#endif
#ifdef SYS_setpgid
  case SYS_setpgid:
#endif
#ifdef SYS_setpriority
  case SYS_setpriority:
#endif
#ifdef SYS_setregid
  case SYS_setregid:
#endif
#ifdef SYS_setregid32
  case SYS_setregid32:
#endif
#ifdef SYS_setresgid
  case SYS_setresgid:
#endif
#ifdef SYS_setresgid32
  case SYS_setresgid32:
#endif
#ifdef SYS_setresuid
  case SYS_setresuid:
#endif
#ifdef SYS_setresuid32
  case SYS_setresuid32:
#endif
#ifdef SYS_setreuid
  case SYS_setreuid:
#endif
#ifdef SYS_setreuid32
  case SYS_setreuid32:
#endif
#ifdef SYS_setrlimit
  case SYS_setrlimit:
#endif
#ifdef SYS_set_robust_list
  case SYS_set_robust_list:
#endif
#ifdef SYS_setsid
  case SYS_setsid:
#endif
#ifdef SYS_set_thread_area
  case SYS_set_thread_area:
#endif
#ifdef SYS_set_tid_address
  case SYS_set_tid_address:
#endif
#ifdef SYS_setuid
  case SYS_setuid:
#endif
#ifdef SYS_setuid32
  case SYS_setuid32:
#endif
#ifdef SYS_sigaction
  case SYS_sigaction:
#endif
#ifdef SYS_sigaltstack
  case SYS_sigaltstack:
#endif
#ifdef SYS_signal
  case SYS_signal:
#endif
#ifdef SYS_signalfd
  case SYS_signalfd:
#endif
#ifdef SYS_signalfd4
  case SYS_signalfd4:
#endif
#ifdef SYS_sigpending
  case SYS_sigpending:
#endif
#ifdef SYS_sigprocmask
  case SYS_sigprocmask:
#endif
#ifdef SYS_sigreturn
  case SYS_sigreturn:
#endif
#ifdef SYS_sigsuspend
  case SYS_sigsuspend:
#endif
#ifdef SYS_socketpair
  case SYS_socketpair:
#endif
#ifdef SYS_stat
  case SYS_stat:
#endif
#ifdef SYS_stat64
  case SYS_stat64:
#endif
#ifdef SYS_statfs
  case SYS_statfs:
#endif
#ifdef SYS_statfs64
  case SYS_statfs64:
#endif
#ifdef SYS_sysfs
  case SYS_sysfs:
#endif
#ifdef SYS_sysinfo
  case SYS_sysinfo:
#endif
#ifdef SYS_time
  case SYS_time:
#endif
#ifdef SYS_timer_create
  case SYS_timer_create:
#endif
#ifdef SYS_timer_delete
  case SYS_timer_delete:
#endif
#ifdef SYS_timerfd_create
  case SYS_timerfd_create:
#endif
#ifdef SYS_timerfd_gettime
  case SYS_timerfd_gettime:
#endif
#ifdef SYS_timerfd_settime
  case SYS_timerfd_settime:
#endif
#ifdef SYS_timer_getoverrun
  case SYS_timer_getoverrun:
#endif
#ifdef SYS_timer_gettime
  case SYS_timer_gettime:
#endif
#ifdef SYS_timer_settime
  case SYS_timer_settime:
#endif
#ifdef SYS_times
  case SYS_times:
#endif
#ifdef SYS_ugetrlimit
  case SYS_ugetrlimit:
#endif
#ifdef SYS_ulimit
  case SYS_ulimit:
#endif
#ifdef SYS_umask
  case SYS_umask:
#endif
#ifdef SYS_uname
  case SYS_uname:
#endif
#ifdef SYS_unshare
  case SYS_unshare:
#endif
#ifdef SYS_uselib
  case SYS_uselib:
#endif
#ifdef SYS_ustat
  case SYS_ustat:
#endif
#ifdef SYS_wait4
  case SYS_wait4:
#endif
#ifdef SYS_waitid
  case SYS_waitid:
#endif
#ifdef SYS_waitpid
  case SYS_waitpid:
#endif
    return deny_all;

#ifdef SYS_exit
  case SYS_exit:
#endif
#ifdef SYS_exit_group
  case SYS_exit_group:
#endif
    /* Special case: exiting is allowed, even in -e mode,
       but the exit status is forced to 0. */
    SET_REG(pid, ARG1, 0);
    return 0;

#ifdef SYS_fcntl
  case SYS_fcntl:
#endif
#ifdef SYS_fcntl64
  case SYS_fcntl64:
#endif
    /* Special case: fcntl is allowed, but only for the *FD and *FL
       operations.  This is a compromise between not allowing it at
       all, which would break some interpreters, and trying to go
       through the dozens of extended ops and figure out which ones
       can affect global state.  */
    {
      int cmd = GET_REG(pid, ARG2);
      if (cmd == F_DUPFD || cmd == F_DUPFD_CLOEXEC ||
          cmd == F_GETFD || cmd == F_SETFD || cmd == F_SETFL || cmd == F_GETFL)
        return deny_all;
    }
    return 1;

#ifdef SYS_kill
  case SYS_kill:
#endif
#ifdef SYS_rt_sigqueueinfo
  case SYS_rt_sigqueueinfo:
#endif
#ifdef SYS_rt_tgsigqueueinfo
  case SYS_rt_tgsigqueueinfo:
#endif
#ifdef SYS_tkill
  case SYS_tkill:
#endif
#ifdef SYS_tgkill
  case SYS_tgkill:
#endif
    /* Special case: kill is allowed if and only if directed to the calling
       process. */
    {
      pid_t kpid = GET_REG(pid, ARG1);
      if (kpid == pid)
        return deny_all;
    }
    return 1;

#ifdef SYS_mmap
  case SYS_mmap:
#endif
#ifdef SYS_mmap2
  case SYS_mmap2:
#endif
    /* Special case: mmap is allowed if it is private or read-only.  */
    {
      int prot  = GET_REG(pid, ARG3);
      int flags = GET_REG(pid, ARG4);
      if ((flags & (MAP_SHARED|MAP_PRIVATE)) == MAP_PRIVATE)
        return deny_all;
      if (!(prot & PROT_WRITE))
        return deny_all;
    }
    return 1;

    /* Special case: open() variants are allowed only if read-only and
       not creating. */
#ifdef SYS_open
  case SYS_open:
#endif
#ifdef SYS_openat
  case SYS_openat:
#endif
#ifdef SYS_open_by_handle_at
  case SYS_open_by_handle_at:
#endif
    {
      int flags = ((scnum == SYS_open)
                   ? GET_REG(pid, ARG2)
                   : GET_REG(pid, ARG3));
      if (!(flags & O_CREAT) && ((flags & O_ACCMODE) == O_RDONLY))
        return deny_all;
    }
    return 1;

#ifdef SYS_write
  case SYS_write:
#endif
#ifdef SYS_write64
  case SYS_write64:
#endif
#ifdef SYS_writev
  case SYS_writev:
#endif
#ifdef SYS_pwrite
  case SYS_pwrite:
#endif
#ifdef SYS_pwrite64
  case SYS_pwrite64:
#endif
#ifdef SYS_pwritev
  case SYS_pwritev:
#endif
    /* Special case: optionally, the program is allowed to write to
       stderr.  This opens a gaping hole in the policy, but it can be
       quite entertaining to watch programs moan about how nothing works. */
    if (allow_stderr) {
      int fd = GET_REG(pid, ARG1);
      if (fd == 2)
        return 0;
    }
    return 1;

  default:
    /* All other system calls are unconditionally denied. */
    return 1;
  }
}

static void
usage(char *progname)
{
  fprintf(stderr, "usage: %s [-adeS] program args...\n", progname);
  fputs("\t-a  log allowed system calls\n"
        "\t-d  log denied system calls\n"
        "\t-e  deny everything, not just output\n"
        "\t-S  permit writes to stderr\n", stderr);
  exit(2);
}

int
main(int argc, char **argv)
{
  pid_t pid;
  int   status;
  int   opt;
  long  last_syscall = SYS_unimplemented;
  int   last_allowed = 0;
  int   after_execve = 0;
  int   trace_active = 0;
  int   allow_stderr = 0;
  int   deny_all     = 0;
  int   log_allowed  = 0;
  int   log_denied   = 0;

  while ((opt = getopt(argc, argv, "+adeS")) != -1) {
    switch (opt) {
    case 'a': log_allowed  = 1; break;
    case 'd': log_denied   = 1; break;
    case 'e': deny_all     = 1; break;
    case 'S': allow_stderr = 1; break;
    default:
      usage(argv[0]);
    }
  }
  if (optind == argc) {
    usage(argv[0]);
  }

  setvbuf(stdout, 0, _IOLBF, 0);
  setvbuf(stderr, 0, _IOLBF, 0);

  pid = fork();
  if (pid == -1) {
    perror("fork");
    exit(1);

  } else if (pid == 0) {
    raise(SIGSTOP); /* synch with parent */
    execvp(argv[optind], argv+optind);
    perror("execvp");
    exit(1);
  }

  /* If we get here, we are the parent. */
  for (;;) {
    pid_t rv = waitpid(pid, &status, WUNTRACED);
    if (rv != pid) {
      perror("waitpid");
      kill(pid, SIGKILL);
      exit(1);
    }
    if (!WIFSTOPPED(status)) {
      if (WIFEXITED(status))
        printf("Program exited, status = %d\n", WEXITSTATUS(status));
      else if (WIFSIGNALED(status))
        printf("Program killed by signal %d\n", WTERMSIG(status));
      else {
        printf("Un-decodable status %04x\n", status);
        kill(pid, SIGKILL); /* just in case */
      }
      exit(0);
    }
    if (WSTOPSIG(status) == SIGSTOP && !trace_active) {
      /* This is the raise(SIGSTOP) on the child side of the fork. */
      trace_active = 1;
      xptrace(PTRACE_SEIZE, pid, 0, (void*)PTRACE_O_TRACESYSGOOD);
      xptrace(PTRACE_SYSCALL, pid, 0, 0);
    }
    else if (WSTOPSIG(status) == (SIGTRAP|0x80)) {
      if (last_syscall == SYS_unimplemented) {
        last_syscall = GET_REG(pid, NUMBER);
        /* The child process is allowed to execute normally until an
           execve() succeeds.  */
        if (after_execve && deny_syscall(pid, last_syscall,
                                         deny_all, allow_stderr)) {
          last_allowed = 0;
          SET_REG(pid, NUMBER, SYS_unimplemented);
        } else {
          last_allowed = 1;
          if (log_allowed) {
            /* Log this now, we may not get another chance. */
            printf("syscall %ld...\n", last_syscall);
          }
        }
      } else {
        if (last_allowed ? log_allowed : log_denied) {
          long scret = GET_REG(pid, RESULT);
          printf("syscall %ld%s = %ld\n",
                 last_syscall, last_allowed ? "" : " (denied)", scret);
        }
        if (last_allowed && (last_syscall == SYS_execve ||
                             last_syscall == SYS_execveat)) {
          long scret = GET_REG(pid, RESULT);
          if (scret == 0)
            after_execve = 1;
        }
        last_syscall = SYS_unimplemented;
      }
      xptrace(PTRACE_SYSCALL, pid, 0, 0);
    }
    else if (WSTOPSIG(status) == SIGTRAP) {
      /* Swallow all SIGTRAPs, they are probably spurious debug events. */
      xptrace(PTRACE_SYSCALL, pid, 0, 0);
    } else {
      /* Allow all normal signals to proceed unmolested. */
      if (log_allowed) {
        printf("process received signal %d\n", WSTOPSIG(status));
      }
      xptrace(PTRACE_SYSCALL, pid, 0, (void*)(uintptr_t)WSTOPSIG(status));
    }
  }
}

Answer 12

TeX

\catcode`\\=10

I'm not sure this will actually work, but in theory this should break \ as escape character leaving you no way to fix it. Normally TeX can read and write files, now it can't write anything that depends on logic. Thus the language is now broken as defined by OP.

EDIT: Other kill commands taken from the comments (although both might violate the code-must-be-executed rule):

• \def\fi{}\iffalse by smpl creates an un-closable if branch
• \catcode13=9% by iwillnotexist idonotexist creates a never-ending comment

Answer 13

Scratch

The when [timer v] > (0) will run as soon as the code is initialised, which if you're in the editor is before you even start the code. The when I receive (join[][]) will cause an error to be thrown every time anything is broadcast, pausing code execution if you have Developer version of Flash. The break function will create clones, and trigger the broadcast error. Every single clone will last two seconds then delete itself, putting strain on the stack. And every clone will respond to the when [timer v] > (0), running the break subroutine and resetting the timer, which causes the timer code to be run again. Also, each clone will respond to every broadcast error as well, meaning the number of errors per evaluation of break is the number of clones squared. Did I forget to mention that the break function has run without screen refresh checked, causing the editor to freeze, jolt and lag, as well as the grabbing and allocating memory. And maxing out the CPU.

Any code added anywhere with this running will find itself unable to create clones (300 clone limit surpassed) as well as heating up and crashing the computer running it. And grabbing memory until there is no more to grab, leaving variables misbehaving.

And, after there's too much lag to trigger the when [timer v] > (0) block, it'll still be running break.

Thanks to @towerofnix for reminding me about the when I receive glitch I found a while back, and giving me the idea for run without screen refresh. If you liked this, here's the original: https://codegolf.stackexchange.com/a/61357/43394

Answer 14

Mathematica / Wolfram Language

Mathematica is an interpreted language in which command names are symbols that can be manipulated by the programmer. You can't delete built-in operators, but you can overload them or otherwise modify their function. The following scrambles the "With" command, which is needed for assignment to variables even internally. This change prevents the kernel from holding arguments unevaluated until the assignment is complete, and it kills the language quite dead.

ClearAttributes["With", HoldAll]

If this command is run in an interactive session or within a block of code, Mathematica will not even be able add 1+1 (the resulting error message is about a page long so I won't include it here).

Answer 15

PHP

I'm suprised that it actually works, but closing STDOUT and STDERR suppresses all output. To be sure that they will not be opened again, we open /dev/null three times to reassign the file descriptors 0, 1 and 2:

<?php
fclose(STDIN);
fclose(STDOUT);
fclose(STDERR);
fopen('/dev/null','r');
fopen('/dev/null','w');
fopen('/dev/null','w');

// insert program here

More on that: https://stackoverflow.com/questions/937627/how-to-redirect-stdout-to-a-file-in-php

Answer 16

DOS batch (prior to Windows 95 I believe)

CTTY

Issued with no arguments, this disconnects the command line from the terminal. Any further attempts to read input or generate output don't do anything.

In case you wanted to know how to use CTTY correctly:

MODE COM1,8600,8,N,1
CTTY COM1

A slightly more powerful batch file could even answer the modem and connect whatever dialed in to CTTY.

Answer 17

Common Lisp

(set-macro-character #\( (lambda (x y) ()))

I hope you didn't need those opening parentheses.

This is a reader macro that tells the Lisp Reader to replace each instance of ( with a call to (lambda (x y) ()), a function that takes two arguments and returns nothing. So, for example, it would read (foo) as foo), interpret foo as a variable and then throw an unmatched parenthesis error on 0.

Answer 18

Scratch

Here's a pretty simple example that will crash your browser (and, in theory, your computer):

I left this running for about twenty seconds, then lost 2.65 GB of memory to Scratch. Only a moment later and 5 GB were gone.

I highly suggest you have a means to force quit either Adobe Flash or your web browser before running this!

---

^{I really wanted to make a cool answer like the clear(this) JS one but sadly Scratch doesn't have any ways to do that. Feel free to update this post (or make your own) if you DO find another way to make Scratch unusable though!}

Answer 19

Thue

::=

With a newline at the end

The thue language relies on defining rulesets and a ::= denotes the end of the ruleset. It is impossible to do ANYTHING in thue without defining rules that do it, so regardless of what you put after the ::=, nothing can happen.

Alternative answer

A::=
B::=
C::=
D::=
E::=
F::=
G::=
H::=

(and so on for the every character in all of Unicode including those before the A character and non-printable characters). This requires the command-line option -r.

Answer 20

Befunge-96

'~h

The user's code can follow anywhere after this sequence, as long as these are the first three characters in the source.

The ' command (one-shot string mode) pushes the ASCII value of the ~ onto the stack (i.e. 126), and the h command then sets what is known as the Holistic Delta with that value. For those not familiar with Befunge-96, the Holistic Delta is an offset that is added to the value of every command byte that the interpreter encounters.

Once the delta is set to 126, the only valid command that can be generated is ~ (character input), via a null byte in the source. Anything other than a null byte would translate to a value greater than 126, and none of those values would be valid Befunge commands.

I think it's safe to say that this would make it ineligible to qualify as a programming language.

Answer 21

MATLAB

The following piece of code makes the environment completely unusable¹:

builtin = @(varargin)false; clear = @(varargin)false;
%// Insert code after this point

This overrides the builtin function and the clear function with new anonymous function handles that simply return false every time you try and call these functions. The builtin function ensures that if there are any custom functions you write in MATLAB that are the same name as those that are built-in to MATLAB (things like sum, max, min, etc.), you are able to call these unambiguously instead of the overloaded functions. Similarly, clear gives you the ability to clear all variables that are currently declared so you can start anew. By removing these capabilities, there is no way that you can use MATLAB unless you restart the program.

In MATLAB R2015a, I also get the following message:

The Workspace are the variables that are currently declared in the environment so that you can use them for later. This permanently disables the Workspace, so any variables you try and create will not be saved and hence no progress can be made when executing lines of code in MATLAB.

^{1: Credit goes to user Dev-iL who originally discovered the idea.}

Answer 22

///

/\///

The only operation in /// is repeated string substitution, like this: /pattern/replacement/.

This code removes every /, that way you can't use repeated string substitution, so basically everything you write after that will get printed (except for /s).

You can still use \s, but that won't help you much.

Answer 23

Boo

macro harmless:
    Context.Parameters.Pipeline.Clear()

And then, somewhere else in the project,

harmless

A simple macro with a harmless-sounding name, but an amazingly frustrating effect. The Boo compiler uses a multi-step pipeline that begins with parsing source into an AST and ends with code generation. (Generally. It can be reconfigured for various applications.) Every step in between performs various operations on the AST.

Partway through is the macro expansion stage, in which macros are executed in the context of the compiler. Remember the bit in the last paragraph, about the pipeline being reconfigurable? If, during macro expansion, you invoke a macro that clears the pipeline, no error will be displayed to the user, but all steps after macro expansion (including code generation) are no longer there. So you end up with something that looks like a successful compilation--no error messages displayed--but for some reason there's no binary produced! Guaranteed to drive even the best troubleshooters up the wall, if you hide the macro and the invocation well.

Answer 24

NGN/APL

NGN/APL allows redefining primitives, so redefining (←) all primitive functions to ⊢ ("pass through": both ⊢3 and 2⊢3 gives 3) makes the language completely useless:

⍪←-←+←?←⍵←∊←⍴←~←↑←↓←⍳←○←*←⌈←⌊←⍕←⊂←⊃←∩←∪←⊥←⊤←|←<←≤←=←≥←>←≠←∨←∧←×←÷←⍒←⍋←⌽←⍉←⊖←⍟←⍱←⍲←!←⌹←⊣←⍎←⊢

Try it here.

Answer 25

Zsh, 6 bytes

set -n

Try it online!

zsh has an option no_exec or -n which permanently disables all commands. I have absolutely no idea why.

Answer 26

Ruby (29 characters)

class Object;def send;end;end

As 'send' is used internally whenever a method is called within Ruby, and since all objects inherit from the Object class. This should stop any method being run.

Fun Fact: This is perfectly sound in theory. But it appears, for some reason, not to hobble the Ruby language. I have no idea why it's possible to run this code and then still use an open Ruby environment.

Answer 27

Taxi, 2354 bytes.

This little program simply runs the taxi in a large joyride through Townsburg, running out of gas. Any code you run after this will quickly error with error: out of gas. And even if you could reach a gas station, which I don't think is possible, you couldn't get any gas, as no money has been collected, since there are no passengers.

Go to Trunkers: west, 1st right, 3rd left, 2nd left, 3rd left, 2nd right, 1st right, 2nd right, 1st right, 1st left, 1st right, 1st left, 3rd left, 2nd left, 3rd left, 2nd right, 1st right, 2nd right, 1st right, 1st left, 1st right, 1st left, 3rd left, 2nd left, 3rd left, 2nd right, 1st right, 2nd right, 1st right, 1st left, 1st right, 1st left, 3rd left, 2nd left, 3rd left, 2nd right, 1st right, 2nd right, 1st right, 1st left, 1st right, 1st left, 3rd left, 2nd left, 3rd left, 2nd right, 1st right, 2nd right, 1st right, 1st left, 1st right, 1st left, 3rd left, 2nd left, 3rd left, 2nd right, 1st right, 2nd right, 1st right, 1st left, 1st right, 1st left, 3rd left, 2nd left, 3rd left, 2nd right, 1st right, 2nd right, 1st right, 1st left, 1st right, 1st left, 3rd left, 2nd left, 3rd left, 2nd right, 1st right, 2nd right, 1st right, 1st left, 1st right, 1st left, 3rd left, 2nd left, 3rd left, 2nd right, 1st right, 2nd right, 1st right, 1st left, 1st right, 1st left, 3rd left, 2nd left, 3rd left, 2nd right, 1st right, 2nd right, 1st right, 1st left, 1st right, 1st left, 3rd left, 2nd left, 3rd left, 2nd right, 1st right, 2nd right, 1st right, 1st left, 1st right, 1st left, 3rd left, 2nd left, 3rd left, 2nd right, 1st right, 2nd right, 1st right, 1st left, 1st right, 1st left, 3rd left, 2nd left, 3rd left, 2nd right, 1st right, 2nd right, 1st right, 1st left, 1st right, 1st left, 3rd left, 2nd left, 3rd left, 2nd right, 1st right, 2nd right, 1st right, 1st left, 1st right, 1st left, 3rd left, 2nd left, 3rd left, 2nd right, 1st right, 2nd right, 1st right, 1st left, 1st right, 1st left, 3rd left, 2nd left, 3rd left, 2nd right, 1st right, 2nd right, 1st right, 1st left, 1st right, 1st left, 3rd left, 2nd left, 3rd left, 2nd right, 1st right, 2nd right, 1st right, 1st left, 1st right, 1st left, 3rd left, 2nd left, 3rd left, 2nd right, 1st right, 2nd right, 1st right, 1st left, 1st right, 1st left, 3rd left, 2nd left, 3rd left, 2nd right, 1st right, 2nd right, 1st right, 1st left, 1st right, 1st left, 3rd left, 2nd left, 3rd left, 2nd right, 1st right, 2nd right, 1st right, 1st left, 1st right, 1st left, 3rd left, 2nd left, 3rd left, 2nd right, 1st right, 2nd right, 1st right, 1st left, 1st right, 1st left, 3rd left, 2nd left, 3rd left, 2nd right, 1st right, 2nd right, 1st right, 1st left, 1st left, 1st left, 1st right.

Answer 28

Tcl

foreach x [info commands] {if {$x!="rename"&&$x!="if"} {rename $x ""}}

This removes all keywords from the language except if and rename.

The above code would cause any new code to error out. So it's debatable if the new inserted code actually gets "executed". Below is a version that executes new code but does nothing because it changes all keywords (except if and proc) to a no-operation:

foreach x [info commands] {if {$x!="if"&&$x!="proc"} {
    proc $x args {}
}}

Instead of deleting keywords this code replaces them with a function that does nothing.

(Note: I'm using "keywords" very loosely here because Tcl doesn't have keywords, only functions)

Answer 29

Factor

UNUSE: syntax

Try it online!

It's not a joke, you can really unload the "built-in" syntax elements. And then you cannot write anything useful: you can't load any library, you can't define a named function or a lambda, you can't take input from stdin (or anything) or print to stdout (or anything). The engine still tries to parse the source code; it just fails because it forgot everything it needs to know.

There is still one thing that works: pushing number literals (which seems to be built into the engine itself). But you can't do any arithmetic or interesting stuff with them.

Answer 30

Ruby 2.2.3

protected_methods = [:instance_methods,:__send__,:remove_method,:include?,:==,:to_s,:call,:pop]
protected_constants = [ObjectSpace, NameError, Exception, Module]
END{exit(0)}
ObjectSpace.each_object(Module) do |mod|
  next if protected_constants.include?(mod)
  im = mod.instance_methods
  while (meth = im.pop)
    next if protected_methods.include? meth
    mod.__send__(:remove_method,meth) rescue NameError
  end
  singleton = (class << mod;self;end)
  singleton_im = singleton.instance_methods
  while (meth = singleton_im.pop) 
    next if protected_methods.include? meth
    singleton.__send__(:remove_method,meth) rescue NameError
  end
end
s = (class << self;self;end)
ms = s.instance_methods
while (m = ms.pop)
  next if protected_methods.include? m
  s.__send__(:remove_method,m)
end

# Your code goes here

Ah, I love ruby. This code iterates through every Module (including classes) and un-defines all the methods, except for the ones in protected_methods. Anything you try to call is undefined, including Object#method_missing which means, due to a ruby quirk, a stackoverflow (technically stack level too deep) error is thrown.

Fun fact: if Exception is not in protected_constants then ruby 2.2.3 segfaults (maybe I should submit a bug report about that /s).

This was surprisingly bigger than I was first expecting, particularly because many things can't be DRY, because that would involve using methods that I need to be undefined.

I am pretty sure this works. I've taken a multi-layered approach in that even if you manage to get something in, you probably won't be able to do anything to it; 5+4 results in undefined method '+' for 5:Fixnum, and good luck getting anything out.

---

In case anyone needs to debug this, or if I forget, you can output after undefining IO#write if you grab a reference to it beforehand:

debug = STDOUT.method(:write)
class IO;remove_method :write;end
debug.call('foo bar')