Using the controller create Event for Evil

This mod has an issue. Can you spot it?

# controller

## create
```
let inst = instance_create_depth(0,0, 0, omod_instance)
inst.persistent = true
```

# instance

## create
```sp
self.name = "Fern"
```

## step
```sp
-- should print "Fern", but it doesn't do anything ???
show_message(self.name)
```

Here’s the issue.

# controller

## create

...

If you make Instances during a controller’s create Event, the instance create Event fails to run. To get around this, you can set an Alarm for one frame later, or use the room_start Event instead.

Using Data Structures as Booleans

Can you spot the issue with this code? Your hint is that global.save_data is a ds_map.

# controller
## room_start
```
if !global.save_data {
  return
}
-- do stuff if a save is loaded
```

The problem is a sneaky combination of Catspeak’s boolean coercion and function pointers. What we want is to quickly determine if global.save_data is a valid ds_map without calling ds_exists, so we abuse the default value (-1) as a boolean (false). Normally this works correctly with no issues, however, GameMaker can assign 0 as a data structure identifier, which is also a false value. The end result is a false negative; the save file is treated as if it doesn’t exist, causing issues.

How do we fix this? In the simple case, we can compare to -1 instead, or just do it the right way.

-- compare to -1
if global.save_data == -1 {
  return
}

-- check if exists
if !ds_exists(global.save_data, ds_type_map) {
  return
}

Creating instances inside with blocks

There’s an issue with this code. Can you spot it?

# controller
## room_start
```sp
with self {
  -- create an instance inside a `with` block
  instance_create_depth(0,0,0, omod_instance)
}
```

# instance
## step
```sp
show_message("Never runs :c")
```

Creating instances (of any kind) while inside a with block is a problem. In RMML 6, this causes an immediate crash and leads to all kinds of funny issues.

This is resolved by later Catspeak releases, however Rusted Moss still uses an affected version.