Introduce -m/--module flag to execute a main function in a package (JuliaLang#52103)

This aims to bring similar functionality to Julia as the `-m` flag for
Python which exists to directly run some function in a package and being
able to pass arguments to that function.

While in Python, `python -m package args` runs the file
`<package>.__main__.py`, the equivalent Julia command (`julia -m Package
args`) instead runs `<Package>.main(args)`. The package is assumed to be
installed in the environment `julia` is run in.

An example usage could be:

Add the package:
```julia
(@v1.11) pkg> add https://github.com/KristofferC/Rot13.jl
     Cloning git-repo `https://github.com/KristofferC/Rot13.jl`
    Updating git-repo `https://github.com/KristofferC/Rot13.jl`
   Resolving package versions...
    Updating `~/.julia/environments/v1.11/Project.toml`
  [43ef800a] + Rot13 v0.1.0 `https://github.com/KristofferC/Rot13.jl#master`
    Updating `~/.julia/environments/v1.11/Manifest.toml`
  [43ef800a] + Rot13 v0.1.0 `https://github.com/KristofferC/Rot13.jl#master`
```

And then it can be run (since it has a `main` function) via:

```
❯ ./julia/julia -m Rot13 "encrypt this for me" "and this as well"
rapelcg guvf sbe zr
naq guvf nf jryy
```

I'm not sure if `-m/--module` is the best choice but perhaps the
association to Python makes it worth it.

Author: KristofferC

NEWS.md

@@ -23,6 +23,9 @@ Compiler/Runtime improvements
 Command-line option changes
 ---------------------------
 
+* The `-m/--module` flag can be passed to run the `main` function inside a package with a set of arguments.
+  This `main` function should be declared using `@main` to indicate that it is an entry point.
+
 Multi-threading changes
 -----------------------
 

base/client.jl

@@ -240,7 +240,7 @@ function exec_options(opts)
         if cmd_suppresses_program(cmd)
             arg_is_program = false
             repl = false
-        elseif cmd == 'L'
+        elseif cmd == 'L' || cmd == 'm'
             # nothing
         elseif cmd == 'B' # --bug-report
             # If we're doing a bug report, don't load anything else. We will
@@ -292,6 +292,13 @@ function exec_options(opts)
         elseif cmd == 'E'
             invokelatest(show, Core.eval(Main, parse_input_line(arg)))
             println()
+        elseif cmd == 'm'
+            @eval Main import $(Symbol(arg)).main
+            if !should_use_main_entrypoint()
+                error("`main` in `$arg` not declared as entry point (use `@main` to do so)")
+            end
+            return false
+
         elseif cmd == 'L'
             # load file immediately on all processors
             if !distributed_mode

doc/man/julia.1

@@ -106,6 +106,11 @@ Enable or disable usage of native code caching in the form of pkgimages
 -e, --eval <expr>
 Evaluate <expr>
 
+.TP
+-m, --module <Package> [args]
+Run entry point of `Package` (`@main` function) with `args'.
+
+
 .TP
 -E, --print <expr>
 Evaluate <expr> and display the result

src/jloptions.c

@@ -128,6 +128,8 @@ static const char opts[]  =
     // actions
     " -e, --eval <expr>          Evaluate <expr>\n"
     " -E, --print <expr>         Evaluate <expr> and display the result\n"
+    " -m, --module <Package> [args]\n"
+    "                            Run entry point of `Package` (`@main` function) with `args'.\n"
     " -L, --load <file>          Load <file> immediately on all processors\n\n"
 
     // parallel options
@@ -271,7 +273,7 @@ JL_DLLEXPORT void jl_parse_opts(int *argcp, char ***argvp)
            opt_gc_threads,
            opt_permalloc_pkgimg
     };
-    static const char* const shortopts = "+vhqH:e:E:L:J:C:it:p:O:g:";
+    static const char* const shortopts = "+vhqH:e:E:L:J:C:it:p:O:g:m:";
     static const struct option longopts[] = {
         // exposed command line options
         // NOTE: This set of required arguments need to be kept in sync
@@ -284,6 +286,7 @@ JL_DLLEXPORT void jl_parse_opts(int *argcp, char ***argvp)
         { "banner",          required_argument, 0, opt_banner },
         { "home",            required_argument, 0, 'H' },
         { "eval",            required_argument, 0, 'e' },
+        { "module",          required_argument, 0, 'm' },
         { "print",           required_argument, 0, 'E' },
         { "load",            required_argument, 0, 'L' },
         { "bug-report",      required_argument, 0, opt_bug_report },
@@ -421,6 +424,7 @@ JL_DLLEXPORT void jl_parse_opts(int *argcp, char ***argvp)
         case 'e': // eval
         case 'E': // print
         case 'L': // load
+        case 'm': // module
         case opt_bug_report: // bug
         {
             size_t sz = strlen(optarg) + 1;
@@ -434,6 +438,10 @@ JL_DLLEXPORT void jl_parse_opts(int *argcp, char ***argvp)
             ncmds++;
             cmds[ncmds] = 0;
             jl_options.cmds = cmds;
+            if (c == 'm') {
+                optind -= 1;
+                goto parsing_args_done;
+            }
             break;
         }
         case 'J': // sysimage
@@ -886,6 +894,7 @@ JL_DLLEXPORT void jl_parse_opts(int *argcp, char ***argvp)
                       "This is a bug, please report it.", c);
         }
     }
+    parsing_args_done:
     jl_options.code_coverage = codecov;
     jl_options.malloc_log = malloclog;
     int proc_args = *argcp < optind ? *argcp : optind;

test/loading.jl

@@ -1545,3 +1545,8 @@ end
         @test_throws SystemError("opening file $(repr(file))") include(file)
     end
 end
+
+@testset "-m" begin
+    rot13proj = joinpath(@__DIR__, "project", "Rot13")
+    @test readchomp(`$(Base.julia_cmd()) --startup-file=no --project=$rot13proj -m Rot13 --project nowhere ABJURER`) == "--cebwrpg abjurer NOWHERE "
+end

test/project/Rot13/Project.toml

@@ -0,0 +1,3 @@
+name = "Rot13"
+uuid = "43ef800a-eac4-47f4-949b-25107b932e8f"
+version = "0.1.0"

test/project/Rot13/src/Rot13.jl

@@ -0,0 +1,15 @@
+module Rot13
+
+function rot13(c::Char)
+    shft = islowercase(c) ? 'a' : 'A'
+    isletter(c) ? c = shft + (c - shft + 13) % 26 : c
+end
+
+rot13(str::AbstractString) = map(rot13, str)
+
+function (@main)(ARGS)
+    foreach(arg -> print(rot13(arg), " "), ARGS)
+    return 0
+end
+
+end # module Rot13