5. Expressions, calculations, constraints and requirements

What the runtime evaluates, and how each kind of check reports. A constraint declared on a definition is checked against the object carrying it, so instantiate first when you want a verdict about a value rather than about a default.

Expressions

Literals:

attribute x = 42;              // Integer
attribute y = 3.14;            // Real
attribute flag = true;         // Boolean
attribute name = "System";     // String

Operators:

attribute sum = 10 + 5;        // Arithmetic
attribute product = 3 * 7;
attribute comparison = x > 10; // Relational
attribute logic = flag and true; // Boolean

Feature References:

part Wheel {
    attribute diameter = 16.0;
}

part Vehicle {
    part wheel : Wheel;
    attribute wheelDiameter = wheel.diameter; // Feature chain
}

Composite structures

private import ScalarValues::*;

part def Engine {
    attribute power : Real = 200.0;
}

part def Car {
    part engine : Engine {
        :>> power = 250.0;  // Redefine nested feature
    }
}

Instantiate and inspect:

sysml> %instantiate Car
✓ Created instance of Car
  ID: 1
  Use %features Car to inspect

sysml> %features Car
Instance: Car (ID: 1)
Features:
  engine = Instance(ID: 2)
    power = 250.00

Multiplicity

part System {
    part sensors : Sensor[0..10];  // 0 to 10 sensors
    part wheels : Wheel[4];         // Exactly 4 wheels
}

Calculations, constraints and requirements

Calculations:

sysml> calc distance {
  ...>     in x;
  ...>     in y;
  ...>     return (x * x + y * y);
  ...> }
✓ calc distance

sysml> %calc distance 3 4
✓ distance(3, 4)
  = 25

Constraints:

sysml> constraint ValidSpeed {
  ...>     assert 65 > 0;
  ...>     assert 65 <= 120;
  ...> }
✓ constraint ValidSpeed

sysml> %constraint ValidSpeed
✓ Constraint ValidSpeed passed

Requirements:

sysml> requirement SafetyReq {
  ...>     assume 65 > 0;
  ...>     require 100 > 50;
  ...> }
✓ requirement SafetyReq

sysml> %requirement SafetyReq
✓ Requirement SafetyReq satisfied

See examples/repl-behavioral-demo.sysml for comprehensive examples.

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Next: 6. Behavior: actions and state machines.