main cannot set an exit status without writing to stderr, and IO.die codes above 255 wrap (256 exits 0) #1465

What Bend should do

1. A main of type IO(U32) exits with its result:

   import Base
   
   def main() -> IO(U32):
     do IO<U32>:
       IO.print_err("error: unexpected argument '--frob' found")
       return 2

   This should print the line and exit 2, at the point where an IO(Unit) main exits today. Today it compiles, prints the line and exits 0, natively, in compiled JS and under bend status.bend: the result is silently dropped.

2. An exit status outside 0..255, whether from IO.die or from main, should fail with a message and a non-zero status instead of wrapping. Today the OS wraps it modulo 256, so a failure can report success:

   import Base
   
   def main() -> IO(Unit):
     IO.die(Unit, 256, "x")

   $ ./big; echo $?
   x
   0
   

   The same happens with bun big.js and bend big.bend.

Why

Command-line programs signal errors through the exit status: 2 for a usage error, by convention, after a formatted message of several lines. In Bend today, the only way to exit non-zero is IO.die(code, msg), and it always writes msg plus a newline to stderr. #1101 confirms that newline is intended. So a program must either squeeze its whole message into msg, or print the message itself and then accept a stray blank line from IO.die(code, ""). It can never exit non-zero silently, for example after a child process has already reported the error.

The only workaround is a user-written foreign effect that calls exit(). A foreign def makes bend PROOF.bend fail for every law in the import graph (#1463).

Prior art: C int main, Rust fn main() -> ExitCode.

bend 2.0.36 · Darwin arm64 · bun 1.2.23