Comps are specialized Axon functions that implement software components. Components are declared like other Axon functions but use a specialized syntax via the defcomp keyword. Unlike straight functions, components do not have parameters or a return value. Instead data is input and output to the component using cells.

Cells are named variables with arbitrary meta data. Cells may be get/set externally to the component and are used to input/output data to the component's logic. Within the component's do block cells are accessed and assigned as normal variables.


Components are defined with the defcomp keyword, followed by zero or more cell definitions and a do block. Here is a simple component to sum two input numbers:

  inA: {is:^number, defVal:0}
  inB: {is:^number, defVal:0}
  out: {is:^number, ro}
    out = inA + inB

The defcomp keyword starts the definition. Then we define three named cells along with their meta data. The cell meta is defined as a Dict literal where all the values must be literals (cannot be an expression). The do block implements our computation logic. Inside the do block the cells are accessed as normal variables.


The following APIs are used to work with Axon components in Fantom:

  • Comp: instance of component
  • CompDef: definition of component
  • CellDef: definition of component cell
  • AbstractComp: base class for components implemented in Fantom

You may implement custom components in Fantom in a HxLib. Your extension must advertise it contains Fantom components by putting the following line your pod's metadata in

meta = [...
        "skyarc.fantomComps": "true",

Components are created as follows:

  1. Subclass from AbstractComp
  2. Annotate your class with Axon facet for function meta
  3. Declare constructor with one Obj arg
  4. Declare cells as public fields with Cell facet
  5. Implement onRecompute

Here is a simple example:

@Axon { meta=Str<|dis:"Example Add"|> }
class ExampleAddComp : AbstractComp
  new make(Obj init) : super(init) {}

  @Cell { meta=Str<|dis:"A"|> }
  Number a :=

  @Cell { meta=Str<|dis:"B"|> }
  Number b :=

  @Cell { meta=Str<|ro|> }
  Number? out

  override Void onRecompute(AxonContext cx)
    out = a + b

NOTE: the Fantom APIs for components are subject to change and are currently only available in SkySpark.


Comps can be called as normal functions. Pass a Dict with the input cells. The function will run the component's do block and return a Dict with all the cells. If we name our example from above "foo":

foo()                  // yields {inA:0, inB:0, out:0}
foo({inA:2, inB:3})    // yields {inA:2, inB:3, out:5}

Note if you pass no Dict or omit cells then they will default to null or the value of the defVal tag.

NOTE: this calling syntax is subject to change