How to print an object in NQP ? (For debugging purposes)

• It is easy in Raku:

  1. say that is calling gist in its short loop code
  2. dd The tiny Data Dumper as shown in this post

class Toto { has $.member = 42; }
class Titi { has $.member = 41; has $.toto = Toto.new }
my $ti = Titi.new;
say $ti;
# Titi.new(member => 41, toto => Toto.new(member => 42))
dd $ti;
# Titi $ti = Titi.new(member => 41, toto => Toto.new(member => 42))

• It seems more complicated in NQP

class Toto { has $!member; sub create() {$!member := 42}};
class Titi { has $!member; has $!toto; sub create() {$!member := 41; $!toto := Toto.new; $!toto.create; }}
my $ti := Titi.new;
say($ti);

Cannot stringify this object of type P6opaque (Titi)

Of course, no .gist method, the code calls nqp::encode which finally expects a string.

Reducing the problem to an MRE:

class foo {}
say(foo.new); # Cannot stringify ...

Simplifying the solution:

class foo { method Str () { 'foo' } }
say(foo.new); # foo

In summary, add a Str method.

This sounds simple but there's a whole lot of behind-the-scenes stuff to consider/explain.

nqp vs raku

The above solution is the same technique raku uses; when a value is expected by a routine/operation to be a string, but isn't, the language behavior is to attempt to coerce to a string. Specifically, see if there's a Str method that can be called on the value, and if so, call it.

In this case NQP's NQPMu, which is way more barebones than raku's Mu, doesn't provide any default Str method. So a solution is to manually add one.

More generally, NQP is a pretty hostile language unless you know raku fairly well and have gone thru A course on Rakudo and NQP internals.

And once you're up to speed on the material in that course, I recommend you consider the IRC channels #raku-dev and/or #moarvm as your first port of call rather than SO (unless your goal is specifically to increase SO coverage of nqp/moarvm).

Debugging the compiler code

As you will have seen, the NQP code you linked calls .say on a filehandle.

That then calls this method.

That method's body is $str ~ "\n". That code will attempt to coerce $str to a string (just as it would in raku). That's what'll be generating the "Cannot stringify" error.

A search for "Cannot stringify" in the NQP repo only matched some Java code. And I bet you're not running Rakudo on the JVM. That means the error message must be coming from MoarVM.

The same search in the MoarVM repo yields this line in coerce.c in MoarVM.

Looking backwards in the routine containing that line we see this bit:

/* Check if there is a Str method. */
    MVMROOT(tc, obj, {
        strmeth = MVM_6model_find_method_cache_only(tc, obj,
            tc->instance->str_consts.Str);
});

This shows the backend, written in C, looking for and invoking a "method" called Str. (It's relying on an internal API (6model) that all three layers of the compiler (raku, nqp, and backends) adhere to.)

Customizing the Str method

You'll need to customize the Str method as appropriate. For example, to print the class's name if it's a type object, and the value of its $!bar attribute otherwise:

class foo {
  has $!bar;
  method Str () { self ?? nqp::coerce_is($!bar) !! self.HOW.name(self) }
}
say(foo.new(bar=>42)); # 42

Despite the method name, the nqp say routine is not expecting a raku Str but rather an nqp native string (which ends up being a MoarVM native string on the MoarVM backend). Hence the need for nqp::coerce_is (which I found by browsing the nqp ops doc).

self.HOW.name(self) is another example of the way nqp just doesn't have the niceties that raku has. You could write the same code in raku but the idiomatic way to write it in raku is self.^name.