Foreign Functions

While there are existing foreign functions such as $hash and $abs, people sometimes want more functions to be included for specialized computation. scallopy provides such interface and allows user to define foreign functions in Python. Here is an example defining a custom $sum function in Python which is later used in Scallop:

# Create a new foreign function by annotating an existing function with `@scallopy.foreign_function`
# Note that this function has variable arguments!
@scallopy.foreign_function
def my_sum(*args: int) -> int:
  s = 0
  for x in args:
    s += x
  return s

# Create a context
ctx = scallopy.ScallopContext()

# Register the declared foreign function (`my_sum`)
# Note that the function needs to be registered before it is used
ctx.register_foreign_function(my_sum)

# Add some relations
ctx.add_relation("I", (int, int))
ctx.add_facts("I", [(1, 2), (2, 3), (3, 4)])

# Add a rule which uses the registered function!
ctx.add_rule("R($my_sum(a, b)) = I(a, b)")

# Run the context
ctx.run()

# See the result, should be [(3,), (5,), (7,)]
print(list(ctx.relation("R")))

Now we elaborate on how we define new foreign functions in Python.

Function Signature

The annotator @scallopy.foreign_function performs analysis of the annotated Python function and makes sure that it is accepted as a Scallop foreign function. We require that types are annotated on all arguments and the return value. For simplicity, Python types such as int, bool, and str are mapped to Scallop types (and type families) as following:

If one desires to use a more fine-grained type

Argument Types

Optional Arguments

Variable Arguments

Error Handling