Use a function to lookup values in resource definition

[ordnungswidrig]

Motivation #115 (comment)

Use a new function to lookup values from resource definitions to

• ensure uniform invocation of functions that are specified in a resource definition. The signature of the functions must be f :: context-map -> any
• drop premature conversion of the resource map values to functions. This prevents inspection of the resource definition at runtime because values have be replaced by cryptic anonymous functions

Provposed implementation:

(defn lookup-resource [ctx key]
  (let [v (get-in ctx [:resource key])]
    (if (fn? v) (v ctx) v)

I'm not sure about the name and whether (defn lookup-resource [resource key context]) would be more explicit. However, the later variant is redundant because the resource is always available in the context.