Race in Verse | Fortnite Documentation | Epic Developer Community

The race expression is used to run a block of two or more async expressions concurrently (simultaneously). When the fastest expression completes, it “wins the race”. Any remaining “losing” expressions are canceled, then any expression that follows the race is evaluated.

Verse

set WinnerResult = race:
    # All three async functions start at the same time
    AsyncFunctionLongTime()
    AsyncFunctionShortTime()  # This will win and its result is used
    AsyncFunctionMediumTime()
# Next expression is called after the fastest async function completes
# / when the fastest/shortest async function task (AsyncFunctionShortTime()) completes
# and all other async function tasks (AsyncFunctionLongTime(), AsyncFunctionMediumTime()) are canceled.
NextExpression(WinnerResult)

set WinnerResult = race:
    # All three async functions start at the same time
    AsyncFunctionLongTime()
    AsyncFunctionShortTime()  # This will win and its result is used
    AsyncFunctionMediumTime()
# Next expression is called after the fastest async function completes
# / when the fastest/shortest async function task (AsyncFunctionShortTime()) completes
# and all other async function tasks (AsyncFunctionLongTime(), AsyncFunctionMediumTime()) are canceled.
NextExpression(WinnerResult)

The following code shows the syntax for the race expression.

Verse

expression0
race:
    slow-expression
    mid-expression
    fast-expression
expression1

expression0
race:
    slow-expression
    mid-expression
    fast-expression
expression1

The diagram below shows the execution flow of the expressions.

Race Expression Use


Where you can use a `race` expression	Async contexts
Invocation time of the `race` expression	Async
Requirements for `race` code block	The body of the `race` expression must have at least two expressions, and all the expressions must be async.
What the `race` expression does	Similar to `sync`, but cancels all but the “winning” subexpression. If any other expressions complete at the same simulation time as the earlier expression, the first (earlier) expression “wins” and breaks any tie. Any “losing” expression tasks are canceled.
When the `race` expression completes	The `race` is completed when the “winning” expression in the code block has completed. This refers to the fastest, shortest length, first completed, or least amount of time to complete.
When the next expression after `race` starts	Any expression that follows the `race` expression is started once the first expression finishes.
Result of the `race` expression	The result of a `race` is the result of the first completed expression. The result type is the most common compatible type of all expressions in the code block.

This might seem simple, but race is one of the most useful and powerful expressions in the Verse arsenal. It is key to stopping other arbitrarily complex async code in a structured fashion — a form of early exit. It does this in a very clean way by keeping whatever tests are needed to determine when to stop separated from the code that is to be stopped.

Any async expression can be canceled.
Some async expressions, such as an endless loop or Sleep(Inf) will never complete. The only way they can be stopped is to cancel them. This can be a strong strategy when paired with one or more race expressions.
Async expressions will not have a result if they are canceled, so any variable or other expression that depends on a canceled async expression would not be bound.

Need to stop some complex behavior after some amount of time or after some complex sequence of events trigger? Without race, you would normally need to sprinkle tests, such as polling all throughout your complex behavior. With race, you only need to add all stop conditions as sibling subexpressions to the complex behavior.

Verse

race:
    ComplexBehavior() # Could be simple or as complex as a whole game
    Sleep(60.0)       # Timeout after one minute
    EventTrigger()    # Some other arbitrary test that can be used to stop

race:
    ComplexBehavior() # Could be simple or as complex as a whole game
    Sleep(60.0)       # Timeout after one minute
    EventTrigger()    # Some other arbitrary test that can be used to stop

A race result can be used to determine which subexpression finished first, or won the race.

Verse

# Adding a unique result to subexpressions so it can
# be used to determine which subexpression won
Winner := race:
    block:        # task 1
        AsyncFunction1()
        1
    block:        # task 2
        AsyncFunction2a()
        AsyncFunction2b()
        AsyncFunction2c()