Programs that process streams of information are commonly built by assembling reusable information-flow components. In some systems the components must be chosen from a pre-defined set of primitives; in others the programmer can create new custom components using a general-purpose programming language. Neither approach is ideal: restricting programmers to a set of primitive components limits the expressivity of the system, while allowing programmers to define new components in a general-purpose language makes it difficult or impossible to reason about the composite system. We advocate defining information-flow components in a domain-specific language (DSL) that enables us to infer the properties of the components and of the composed system; this provides us with a good compromise between analysability and expressivity. This paper presents DirectFlow, which comprises a DSL, a compiler and a runtime system. The language allows programmers to define objects that implement information-flow components without specifying how messages are sent and received. The compiler generates Java classes by inferring the message sends and methods, while the run-time library constructs information-flow networks by composition of DSL-defined components with standard components.