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Types, variables, verbs

With components and subcomponents, the specification merely lays out the hierarchical component structure, but tells us little about what needs to go on inside the system. We would like to start expressing this using requirements and constraints on various variables in our design space, but we need to define those first! Let us say we want to specify the torque that our drive mechanism has to supply to the pump:

world.esl
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define type
  MechanicalEnergyFlow is a real with unit Nm


define verbs
  provide to
  convert into


world
  variables
    torque is a MechanicalEnergyFlow
world.esl
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# ---8<--- [start:highlight]
define type
  MechanicalEnergyFlow is a real with unit Nm


define verbs
  provide to
  convert into


world
  variables
    torque is a MechanicalEnergyFlow
# ---8<--- [end:highlight]

  components
    pump is a CentrifugalPump
    drive-mechanism is an ElectricalDriveMechanism


define component CentrifugalPump
  empty


define component ElectricalDriveMechanism
  components
    electric-motor is a BrushlessMotor
    power-source is a Battery
    power-swith is a Switch


define component BrushlessMotor
  empty


define component Battery
  empty


define component Switch
  empty

Types and variables

We first defined a MechanicalEnergyFlow type that will only allow values with the Newton meter (Nm) unit, and instantiated a variable on the global (world) level of that type named torque. Similar to the component instantiation from before, you can use a [name] is a [defined type] to describe this. There are several built-in types, being:

boolean

Boolean values (true or false)

integer

integer valued numbers (1, 2, 3, etc.)

real

real valued numbers (1.0, -2.6234, etc.)

string

literals ("running", "blue", etc.)

The built-in types can always be used without any explicit definition. This is fine, for example:

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world
  variables
    foo is a real

Verbs

We also defined two verbs that we would like to use when we start expressing requirements. It consists of a verb like provide and a preposition like to in this example. By having to stick to a defined set of verbs and prepositions, the ambiguity in your specifications can be kept to an absolute minimum!

Tip

Most of the time, you can start off with the verbs provide to and convert into. provide to for goal requirements and convert into for transformations. We'll explore these in the next pages!

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