Guard Conditions in SysML v2¶

A guard condition is a boolean expression that controls whether a state machine transition fires. The condition must evaluate to true for the transition to occur.

Canonical Keyword: `if`¶

The official OMG SysML v2 specification uses the keyword if for guard conditions. The word guard exists only as a metamodel enum value name (TransitionFeatureKind::guard) — it is not a keyword in the concrete syntax.

Correct syntax¶

transition <name> first <source> [accept <trigger>] [if <condition>] [do <effect>] then <target>;

Mandatory keyword order¶

transition keyword
first <source-state>
accept <trigger> (optional — must come before if)
if <guard-expression> (optional — must come after accept and before then)
do <effect> (optional)
then <target-state>

Examples¶

Guard with signal trigger and effect¶

From the official OMG Annex A Vehicle Model:

transition off_To_starting
    first off
    accept ignitionCmd : IgnitionCmd via ignitionCmdPort
        if ignitionCmd.ignitionOnOff == IgnitionOnOff::on and brakePedalDepressed
    do send new StartSignal() to controller
    then starting;

Guard with trigger only¶

From the OMG validation examples:

transition 'off-starting'
    first off
    accept 'Vehicle Start Signal'
    if vehicle1_c1.'brake pedal depressed'
    do send new 'Start Signal'() to vehicle1_c1.vehicleController
    then starting;

Guard-only transition (no trigger, no effect)¶

accept SetSpeed if currentSpeed >= minSpeed then cruising;

Guard with compound condition¶

From Advent of SysML v2:

accept StartMission via hmi
if reindeerCount == 9 and totalPower >= 36 [SI::kW]
do send new Command("Gee-up!") via eyelet
then mission;

Guard on a trigger-less transition¶

transition first focus if focus.image.isFocused then shoot;

Change trigger (not a guard, but achieves similar effect)¶

accept when currentSpeed < targetSpeed - tolerance then accelerating;

Action-level conditional succession¶

Guard conditions also work at the action flow level, not just state transitions:

if condition then nextAction;

Common Mistakes¶

Mistake 1: Using `guard` instead of `if`¶

// WRONG — 'guard' is not a keyword in the spec
transition first S1 guard condition then S2;

// CORRECT
transition first S1 if condition then S2;

(The sysmlpy parser accepts both if and guard as a compatibility extension, but the canonical keyword is if.)

Mistake 2: Putting `if` after `then`¶

// WRONG — guard must come BEFORE the target
transition first S1 then condition guard S2;

// CORRECT — guard comes before then
transition first S1 if condition then S2;

Mistake 3: Putting `accept` after `if`¶

// WRONG — accept before if per spec
transition first off if enabled accept TurnOn then on;

// CORRECT
transition first off accept TurnOn if enabled then on;

References¶

OMG SysML v2 specification, Section 8.2.2.18.3 "Transition Usages"
OMG Issue SYSML2_-411 — confirms accept must precede if in concrete syntax
OMG SysML v2 Release: https://github.com/Systems-Modeling/SysML-v2-Release
Advent of SysML v2: https://github.com/sensmetry/advent-of-sysml-v2
Sensmetry SysML Cheatsheet: https://sensmetry.com/sysml-cheatsheet/