Requirements and providing them

require(T) reads a dependency and records it in the R channel; composing effects unions their requirements, exactly like errors. the runners only accept Effect[A, E], so running an effect with unmet requirements is a type error, not a runtime surprise.

@dataclass(frozen=True)
class Db:
    url: str
 
 
needs_db = E.require(Db).map(lambda db: db.url)  # Effect[str, Never, Db]
 
program
 = needs_db.provide(Db)(Db("postgres://x"))  # Effect[str] — runnable
 
E.run_sync(program)  # "postgres://x"

provide(T)(impl) subtracts the provided type from R, so requirements can be provided one at a time, anywhere in the program — a partially provided effect is an ordinary value carrying the remainder in R, and the runners accept it only once R reaches Never.

More examples: test_run_sync.py.