Subtype Incompatibility Error with Predicate Term Types

[mgkwon]

Subject of the issue

When a predicate is defined with a type and is called in other places such as an action precondition with a subtype as a term it fails due to the assert in: Predicate.__call__

I just started getting my hands on PDDL so I'm not sure if it is a good practice to utilize the hierarchy of the types. However, PDDL allows me so it should also be allowed. Please let me know what you think.

Your environment

• OS: Ubuntu Focal
• Python version: 3.8.10
• Package Version 0.4.0

Steps to reproduce

1. Define a predicate, at, that expects two terms of type location.
2. Define a variable, l_city, of type city.
3. Attempt to use the at predicate with the term l_city.

p = Variable("p", ["person"])
l = Variable("l", ["location"])
l_city = Variable("l_city", ["city"])
at= Predicate("at", p, l)
...
work = Action(
    "work",
    parameters=[p, l_city],
    precondition=**at(p, l_city)**,
    effect=working(p),
)

Expected behaviour

The at predicate should allow the use of the l_city term because city is a subtype of location in the type hierarchy.

Actual behaviour

The library raises an error, indicating that the predicate terms' types are incorrect. It does not recognize that city is a subtype of location in the type hierarchy, resulting in an incorrect type check. AssertionError: Types of replacements is not correct.

[marcofavorito]

@mgkwon thank you very much for your interest in using pddl. I'm going to have a look at your issue and provide a fix asap!

[fenjenahwe]

Hi! I'm facing the same issue and was wondering if there are any updates regarding this. Is there any quick workaround to make subtypes compatible with predicates?

On a related note, I'm searching for the correct way to declare subtypes when defining the domain, if that is supported by the library. Any tips would be appreciated!

For example:

 requirements = [Requirements.STRIPS, Requirements.TYPING, Requirements.NEG_PRECONDITION]
    domain = Domain("dining-general",
    requirements=requirements,
    types=["objects location hypothesis - world", "agent furniture consumable - objects"],
    predicates=[...],
    actions=[...])

returns a type parsing error. Thanks!

[haz]

Hrmz...from the first example, it doesn't look like you have any sub-type information put in. Here's an example of how sub-typing can be done:

• https://github.com/AI-Planning/pddl/blob/8e3be082c0c2142288ffb14a9766f66bd9b1febf/tests/test_domain.py#L214

That said, it doesn't look like the Variable object is associated with the domain (similar with the predicate).

@marcofavorito should the Domain object (containing type hierarchy) be used in some way for the validation of parameters?

[marcofavorito]

Hi,

@fenjenahwe the types argument in the Domain class constructor is of type Dict[namelike, Optional[namelike], see https://github.com/AI-Planning/pddl/blob/8e3be082c0c2142288ffb14a9766f66bd9b1febf/pddl/core.py#L53C19-L53C68

Hence, in your example, you should write:

types={
    "objects": "world",
    "location": "world",
    "hypothesis": "world",
    "agent": "objects",
    "furniture": "objects",
    "consumable": "objects",
}

Regarding @mgkwon's issue, yes there should be a check in the Domain instantiation. Will try to provide a fix (this time for real :sweat_smile: ).

[fenjenahwe]

Thanks @haz While going through the code I stumbled upon the syntax you shared, but if I use it, and assuming I want A to be a subtype of B, and B to be a subtype of C, then it translates it simply into:

(define (domain dining-general)
    (:requirements :negative-preconditions :strips :typing)
    (:types A B C)

My understanding is that it should translate into:

(define (domain dining-general)
    (:requirements :negative-preconditions :strips :typing)
    (:types A - B - C)

the dash symbolizing a subtype relation (?) - unless I have misunderstood this (if so, would appreciate any resource I can look into to understand this).

For the second point, yes I believe this is the main issue - even when I manage to have a type hierarchy (cf. below), if a Predicate's parameter is supposed to have type C, then a parameter of type B will be rejected and return an error because it is understood as a separate/independent type (regardless of type hierarchy in the domain).

My current workaround (yikes, brace yourselves): I've edited the regex used to parse the types to include the space. This allows me to express the type hierarchy with dashes. https://github.com/AI-Planning/pddl/blob/8e3be082c0c2142288ffb14a9766f66bd9b1febf/pddl/custom_types.py#L30

Then disabled ( :grin: ) the lines that validate the types used in Predicates. https://github.com/AI-Planning/pddl/blob/8e3be082c0c2142288ffb14a9766f66bd9b1febf/pddl/logic/predicates.py#L73-L76

Doing this, I obtained a correct PDDL description of the domain/problem and was solvable.

[fenjenahwe]

Hi,

@fenjenahwe the types argument in the Domain class constructor is of type Dict[namelike, Optional[namelike], see https://github.com/AI-Planning/pddl/blob/8e3be082c0c2142288ffb14a9766f66bd9b1febf/pddl/core.py#L53C19-L53C68

Hence, in your example, you should write:

types={
    "objects": "world",
    "location": "world",
    "hypothesis": "world",
    "agent": "objects",
    "furniture": "objects",
    "consumable": "objects",
}

Regarding @mgkwon's issue, yes there should be a check in the Domain instantiation. Will try to provide a fix (this time for real 😅 ).

Thanks! If do write it this way, I get this:

(define (domain dining-general)
    (:requirements :negative-preconditions :strips :typing)
    (:types agent consumable furniture hypothesis location objects)

My understanding is that these are just types that are independent from each other? Or am I missing something..

[marcofavorito]

Hi, @fenjenahwe the types argument in the Domain class constructor is of type Dict[namelike, Optional[namelike], see https://github.com/AI-Planning/pddl/blob/8e3be082c0c2142288ffb14a9766f66bd9b1febf/pddl/core.py#L53C19-L53C68 Hence, in your example, you should write:

types={
    "objects": "world",
    "location": "world",
    "hypothesis": "world",
    "agent": "objects",
    "furniture": "objects",
    "consumable": "objects",
}

Regarding @mgkwon's issue, yes there should be a check in the Domain instantiation. Will try to provide a fix (this time for real 😅 ).

Thanks! If do write it this way, I get this:

(define (domain dining-general)
    (:requirements :negative-preconditions :strips :typing)
    (:types agent consumable furniture hypothesis location objects)

My understanding is that these are just types that are independent from each other? Or am I missing something..

This is another issue (different from the original one); I will make sure this is documented in another GH issue, and then addressed. Thank you very much for the report!

[marcofavorito]

Regarding @mgkwon's issue, I think a good solution would be to pass the type hierarchy definition inside the Predicate constructor, e.g.:

p = Variable("p", ["person"])
l = Variable("l", ["location"])
l_city = Variable("l_city", ["city"])
at= Predicate("at", p, l, types={...})

Basically, the argument types is of the same Python type as the argument types in the Domain constructor. This adds a bit more complexity in the initialization of Predicate's instances, but I guess it is a necessary evil to preserve the "composability", bottom-up style of our APIs (that is, first I initialize variables, then predicates, etc.).

The change is less straightforward than previously thought, and since a lot work has been done in this work-in-progress PR (https://github.com/AI-Planning/pddl/pull/84) I suggest to include the fix to this issue in that line of work.

[haz]

Should likely have a default for the types parameter so we don't need to specify it, ya?