Finding mappings

Imagine, then, we have an invoice. We moreover have a task that somehow says, "Make a document from this invoice." The way to do that is to find a unit that is indexed as a map from an invoice onto a document. That map satisfies the need to make a document, so we set up the map, let it do its thing, and we now have a document.

Note that the task is thus a high-level description of the essence of the program. Again, this is akin to 4GL languages in that we state what to do, and the semantics tell us how to do it. This is truly declarative programming.