No more non-speaking "Attempt to call nil" messages :)

[Stefan Reich]

Another little idea for LuaJ that I like a lot - "named nils". Don't
you hate those non-speaking "attempt to call nil" messages, too? =)

Add to Lua.java:

    /** disable for more speed */
    public static final boolean useNamedNil = true;

    public static LuaValue namedNil(LuaValue name) {
            return useNamedNil ? new NamedNil(name) : NIL;
    }

Create this class:

package org.luaj.vm2;

public class NamedNil extends LuaNil {
  public LuaValue name;

  public NamedNil(LuaValue name) {
    this.name = name;
  }
}

Change 2 functions in LuaTable:

    public LuaValue rawget( LuaValue key ) {
            if ( key.isinttype() ) {
                    int ikey = key.toint();
                    if ( ikey>0 && ikey<=array.length ) {
                            LuaValue v = m_metatable == null
                                            ? array[ikey-1] : m_metatable.arrayget(array, ikey-1);
                            return v != null ? v : Lua.namedNil(key);
                    }
            }
            return hashget( key );
    }

    protected LuaValue hashget(LuaValue key) {
            if ( hashEntries > 0 ) {
                    for ( Slot slot = hash[ hashSlot(key) ]; slot != null; slot = slot.rest() ) {
                            StrongSlot foundSlot;
                            if ( ( foundSlot = slot.find(key) ) != null ) {
                                    return foundSlot.value();
                            }
                    }
            }
            return Lua.namedNil(key);
    }

Change LuaValue.checkmetatag:

    protected LuaValue checkmetatag(LuaValue tag, String reason) {
            LuaValue h = this.metatag(tag);
            if ( h.isnil() ) {
                    String errorText = reason + typename();
                    if (this instanceof NamedNil)
                            errorText += " (" + ((NamedNil) this).name.tojstring() + ")";
                    throw new LuaError(errorText);
            }
            return h;
    }

And, voila, when running scripts with undefined functions (let's say
loadURL is undefined):

_, _, url = snippetText:find("(http://.*)")
create {type='nt', text=loadURL(url), title='Contents of '..url..' for '..snippetID}

...you get very nice error messages like this:

org.luaj.vm2.LuaError: #546:2 attempt to call nil (loadURL)

Performance hit should be small when turned on - and zero when turned
off and HotSpot-optimized.

Cheers,
Stefan

[gmeqiminfo]

Thank you for this help.

But in our environnement we needed to modify LuaScriptEngine BindingsMetatable class and put this ack to show which function is not defined :

this.rawset(LuaValue.INDEX, new TwoArgFunction() {
    public LuaValue call(LuaValue table, LuaValue key) {

        if (key.isstring()) {
            Object o = bindings.get(key.tojstring());
            if(o == null){
                return Lua.namedNil( key );
            } else {
                return toLua(o);
            }
        }
        else
            return this.rawget(key);
    }
});