State-of-the-art serverless platforms use hardcoded scheduling policies that are unaware of the possible topological constraints of functions. Considering these constraints when scheduling functions leads to sensible performance improvements, e.g., minimising loading times or data-access latencies. This issue becomes more pressing when considered in the emerging multi-cloud and edge-cloud-continuum systems, where only specific nodes can access specialised, local resources. To address this problem, we present a declarative language for defining serverless scheduling policies to express constraints on topologies of schedulers and execution nodes. We implement our approach as an extension of the OpenWhisk platform.