How do I get an unhandled exception to be reported in SML/NJ? - exception

Question

I have the following SML program in a file named testexc.sml:
structure TestExc : sig
val main : (string * string list -> int)
end =
struct
exception OhNoes;
fun main(prog_name, args) = (
raise OhNoes
)
end
I build it with smlnj-110.74 like this:
ml-build sources.cm TestExc.main testimg
Where sources.cm contains:
Group is
csx.sml
I invoke the program like so (on Mac OS 10.8):
sml #SMLload testimg.x86-darwin
I expect to see something when I invoke the program, but the only thing I get is a return code of 1:
$ sml #SMLload testimg.x86-darwin
$ echo $?
1
What gives? Why would SML fail silently on this unhandled exception? Is this behavior normal? Is there some generic handler I can put on main that will print the error that occurred? I realize I can match exception OhNoes, but what about larger programs with exceptions I might not know about?

Answer 1

The answer is to handle the exception, call it e, and print the data using a couple functions available in the system:
$ sml
Standard ML of New Jersey v110.74 [built: Tue Jan 31 16:23:10 2012]
- exnName;
val it = fn : exn -> string
- exnMessage;
val it = fn : exn -> string
-
Now, we have our modified program, were we have the generic handler tacked on to main():
structure TestExc : sig
val main : (string * string list -> int)
end =
struct
exception OhNoes;
open List;
fun exnToString(e) =
List.foldr (op ^) "" ["[",
exnName e,
" ",
exnMessage e,
"]"]
fun main(prog_name, args) = (
raise OhNoes
)
handle e => (
print("Grasshopper disassemble: " ^ exnToString(e));
42)
end
I used lists for generating the message, so to make this program build, you'll need a reference to the basis library in sources.cm:
Group is
$/basis.cm
sources.cm
And here's what it looks like when we run it:
$ sml #SMLload testimg.x86-darwin
Grasshopper disassemble: [OhNoes OhNoes (more info unavailable: ExnInfoHook not initialized)]
$ echo $?
42
I don't know what ExnInfoHook is, but I see OhNoes, at least. It's too bad the SML compiler didn't add a basic handler for us, so as to print something when there was an unhandled exception in the compiled program. I suspect ml-build would be responsible for that task.