Functions

Fault has three kinds of functions: func, sfunc, and unfunc. Because Fault is a specification language and not a programming language, functions aren't about executing instruction but rather defining how the logic might branch out to different scenarios if that function were executed.

Keyword	Where it lives	What it expresses
func	flow{} in .fspec files	A stock value is changed somehow
sfunc	component states in .fsystem files	The state machine transitions
unfunc	flow{} in .fspec files	The preconditions and effects of a black box function

func

The standard function. Used inside flow{} definitions to describe how a flow changes stocks. See Flows for syntax and examples.

sfunc

State functions live inside component definitions and describe what happens when a component is in a particular state. The body may contain advance(), stay(), leave(), flow triggers, and stock conditionals. It may not directly mutate stocks — that is the job of flows. See States for syntax and examples.

unfunc

Short for "unimplemented function." A declarative alternative to func that uses requires and emits clauses instead of imperative steps. The solver uses these clauses to determine when a function can be selected and what it does when it is. unfunc is the primary building block for program synthesis. See Flows for syntax and examples.