First flow

Without knowing anything about flow and its detailed semantics you might be able to guess what this flow below does when executed and what the output to STDOUT will be. First flow It is a fibonacci series generator.

Understanding the flow

NOTE:You can find a complete description of flow semantics in the next section Defining Flows

Root flow

All flows start with a root "flow definition". Other sub-flows can be nested under the root, via references to separate flow description files, to enable encapsulation and flow reuse.

In this case it is the only one, and no hierarchy of flows descriptions is used or needed. You can see the TOML root flow definition for this flow in the flowrcli crate's fibonacci example. root.toml

Interaction with the execution environment

The root defines what the interaction with the surrounding execution environment is, such as Stdout, or any other context function provided by the flow runtime being used (e.g. flowrcli).

The only interaction with the execution environment in this example is the use of stdout to print the numbers in the series to the Terminal.


Functions are stateless, and pure, and just take a set of inputs (one on each of its inputs) and produce an output.

When all the inputs of a function have a value, then the function can run and produce an output, or not produce outputs, as in the case of the impure stdout function.

This flow uses two functions (shown as orange ovals):

  • stdout from the context functions as described above
    • stdout only has one, unnamed, default input and no outputs. It will print the value on STDOUT of the process running the flow runner (flowrcli) that is executing the flow.
  • the add function from the flow standard library flowstdlib to add two integers together.
    • add has two inputs "i1" and "i2" and produces the sum of them on the only, unnamed, "default" output.


Connections (the solid lines) take the output of a function when it has ran, and send it to the input of connected functions. They can optionally have a name.

When a functions has ran, the input values used are made available again at the output.

In this case the following three connections exist:

  • "i2" input value is connected back to the "i1" input.
  • the output of "add" (the sum of "i1" and "i2") is connected back to the "i2" inputs. This connection has optionally been called "sum"
  • the output of "add" (the sum of "i1" and "i2") is connected to the default input of "Stdout". This connection has optionally been called "sum"


Inputs of processes (flows or functions) can be initialized with a value "Once" (at startup) or "Always" (each time it ran) using input initializers (dotted lines)

In this example two input initializers are used to setup the series calculation

  • "Once" initializer with value "1" in the "i2" input of "add"
  • "Once" initializer with value "0" in the "i1" input of "add"