Component Instances

Overview

This guide covers how to create and use library component instances in ParamML - a fundamental feature for building reusable, parametric components.

What you'll learn:

What object instances are
How to create instances
What gets copied to instances
Basic usage patterns
Simple nested instances

For advanced topics (Export dependencies, complex nesting, debugging): → See AI-Export.md

What are Object Instances?

An object instance is created when you use a Library Component as a building block in your own component.

Think of it like:

Component Definition = Blueprint/Template (e.g., "Cube")
Instance = Using that blueprint (e.g., "MyCube")

Analogy

Component Definition (Cube)
    ↓ (instantiate)
Instance (MyCube, YourCube, AnotherCube...)

Like a cookie cutter (component) making multiple cookies (instances).

Creating an Instance

Basic Syntax

To create an instance, use the component name as the T (Type) attribute:

<O N="InstanceName" T="ComponentName">
    <!-- Set input parameters -->
</O>

Simple Example

Component definition:

<O N="Rectangle" T="Project">
    <P N="width" V="100" Role="Input" />
    <P N="height" V="50" Role="Input" />
    
    <O T="Export">
        <P N="area" V="width * height" />
        <P N="perimeter" V="2 * (width + height)" />
    </O>
</O>

Creating instances:

<!-- Instance 1 -->
<O N="SmallRect" T="Rectangle">
    <!-- Inputs set by user -->
    <P N="width" V="50" />
    <P N="height" V="25" />
    
    <!-- Exported parameters copied automatically -->
    <P N="area" V="width * height" />
    <P N="perimeter" V="2 * (width + height)" />
</O>

<!-- Instance 2 -->
<O N="LargeRect" T="Rectangle">
    <!-- Inputs set by user -->
    <P N="width" V="200" />
    <P N="height" V="100" />
    
    <!-- Exported parameters copied automatically -->
    <P N="area" V="width * height" />
    <P N="perimeter" V="2 * (width + height)" />
</O>

Using instances:

<P N="small_area" V="SmallRect.area" />        <!-- = 50 * 25 = 1250 -->
<P N="large_area" V="LargeRect.area" />        <!-- = 200 * 100 = 20000 -->
<P N="total_area" V="small_area + large_area" /> <!-- = 21250 -->

What Gets Copied to an Instance

When you create an instance, only specific content appears:

✅ What Gets Copied

Input parameters (Role="Input")
Everything inside <O T="Export">

❌ What Does NOT Get Copied

Parameters without Role="Input" that are outside Export
Objects outside Export
Geometry (Volume, Surface, Point objects)
Internal calculations outside Export

Example: What Appears

Component definition:

<O N="Beam" T="Project">
    <!-- ✅ Copied: Input parameters -->
    <P N="length" V="1000" Role="Input" />
    <P N="load" V="50" Role="Input" />
    
    <!-- ❌ NOT copied: Internal parameter -->
    <P N="load_factor" V="1.5" />
    
    <!-- ✅ Copied: Export content -->
    <O T="Export">
        <P N="moment" V="load * length * length / 8" />
        <P N="shear" V="load * length / 2" />
    </O>
    
    <!-- ❌ NOT copied: Geometry -->
    <O N="BeamGeo" T="Volume">
        <!-- ... -->
    </O>
</O>

Instance:

<O N="MyBeam" T="Beam">
    <!-- ✅ Inputs appear -->
    <P N="length" V="1000" />
    <P N="load" V="50" />
    
    <!-- ✅ Exports appear -->
    <P N="moment" V="load * length * length / 8" />
    <P N="shear" V="load * length / 2" />
    
    <!-- ❌ load_factor does NOT appear -->
    <!-- ❌ BeamGeo does NOT appear -->
</O>

What you can access:

<P N="m" V="MyBeam.moment" />           <!-- ✅ Works -->
<P N="s" V="MyBeam.shear" />            <!-- ✅ Works -->
<P N="f" V="MyBeam.load_factor" />      <!-- ❌ Not accessible -->
<P N="g" V="MyBeam.BeamGeo" />          <!-- ❌ Not accessible -->

The Export Object

The <O T="Export"> object controls what is accessible when someone creates an instance of your component.

Why Export Exists

Export creates a clean interface - only exposing what users need, hiding implementation details.

Without Export:

❌ Everything would be accessible
❌ Implementation details exposed
❌ Hard to refactor without breaking code
❌ Messy, confusing interface

With Export:

✅ Clean, simple interface
✅ Hide complexity
✅ Can change internals without breaking users
✅ Clear what's intended for external use

Basic Export Example

<O N="Column" T="Project">
    <P N="width" V="400" Role="Input" />
    <P N="height" V="3000" Role="Input" />
    
    <!-- Internal - hidden -->
    <P N="density" V="2400" />
    <P N="unit_weight" V="density * 9.81 / 1000000" />
    
    <!-- Public interface - accessible -->
    <O T="Export">
        <P N="area" V="width * width" />
        <P N="volume" V="area * height" />
        <P N="weight" V="volume * unit_weight" />  <!-- ⚠️ Problem - see AI-Export.md -->
    </O>
    
    <!-- Geometry - hidden -->
    <O N="ColumnGeo" T="Volume">
        <!-- ... -->
    </O>
</O>

Instance sees only:

<O N="MyColumn" T="Column">
    <P N="width" V="400" />
    <P N="height" V="3000" />
    <P N="area" V="..." />
    <P N="volume" V="..." />
    <P N="weight" V="..." />
</O>

⚠️ Note: The weight calculation has a dependency issue. See AI-Export.md for details on the Dependency Rule.

Setting Instance Parameters

Set Input Values

<O N="Beam" T="Project">
    <P N="length" V="1000" Role="Input" />  <!-- Default value -->
    <P N="width" V="200" Role="Input" />
    
    <O T="Export">
        <P N="area" V="length * width" />
    </O>
</O>

<!-- Use defaults -->
<O N="Beam1" T="Beam" />
<!-- Beam1: length=1000, width=200, area=200000 -->

<!-- Set all -->
<O N="Beam3" T="Beam">
    <P N="length" V="2000" />
    <P N="width" V="300" />
</O>
<!-- Beam3: length=2000, width=300, area=600000 -->

Use Expressions

Instance parameters can be expressions:

<P N="column_spacing" V="500" />

<O N="Beam1" T="Beam">
    <P N="length" V="column_spacing * 4" />  <!-- 2000 -->
    <P N="width" V="length / 10" />          <!-- 200 -->
</O>

Reference Other Instances

<O N="Column1" T="Column">
    <P N="height" V="3000" />
</O>

<O N="Beam1" T="Beam">
    <!-- Use Column1's exported value -->
    <P N="length" V="Column1.height / 3" />  <!-- 1000 -->
</O>

Nested Instances (Simple)

Instances can contain other instances.

Simple Nesting

Building block:

<O N="Bolt" T="Project">
    <P N="diameter" V="20" Role="Input" />
    
    <O T="Export">
        <P N="area" V="PI * (diameter/2)^2" />
        <P N="capacity" V="area * 400" />
    </O>
</O>

Component using Bolt:

<O N="Connection" T="Project">
    <P N="num_bolts" V="4" Role="Input" />
    <P N="bolt_diameter" V="20" Role="Input" />
    
    <O T="Export">
        <!-- Create Bolt instance inside Export -->
        <O N="Bolt" T="Bolt">
            <P N="diameter" V="bolt_diameter" />
            <P N="area" V="PI * (diameter/2)^2" />
            <P N="capacity" V="area * 400" />
        </O>
        
        <P N="total_capacity" V="num_bolts * Bolt.capacity" />
    </O>
</O>

Using Connection:

<O N="MyConnection" T="Connection">
    <P N="num_bolts" V="6" />
    <P N="bolt_diameter" V="24" />
    
    <!-- Nested instance appears -->
    <O N="Bolt" T="Bolt" Exported="1">
        <P N="diameter" V="bolt_diameter" />
        <P N="area" V="PI * (diameter/2)^2" />
        <P N="capacity" V="area * 400" />
    </O>
    
    <P N="total_capacity" V="num_bolts * Bolt.capacity" />
</O>

<!-- Can access nested values -->
<P N="bolt_cap" V="MyConnection.Bolt.capacity" />        <!-- ✅ Works -->
<P N="total_cap" V="MyConnection.total_capacity" />      <!-- ✅ Works -->

Key point: The nested Bolt instance must be inside Export for external access.

The `Exported="1"` Attribute

You'll see Exported="1" on nested instances.

How It Works

In component definition (what you write):

<O T="Export">
    <O N="Bolt" T="Bolt">
        <!-- No Exported="1" here -->
    </O>
</O>

In instance (system generates):

<O N="MyConnection" T="Connection">
    <O N="Bolt" T="Bolt" Exported="1">
        <!-- System adds Exported="1" automatically -->
    </O>
</O>

What it means: Marks the object as an exported instance, allowing nested access.

Rule: Never write Exported="1" - the system adds it automatically.

Common Patterns

Pattern 1: Multiple Instances of Same Component

<O N="Column" T="Project">
    <P N="x" V="0" Role="Input" />
    <P N="y" V="0" Role="Input" />
    <P N="height" V="3000" Role="Input" />
    <O T="Export">
        <P N="top_z" V="height" />
        <P N="volume" V="400 * 400 * height" />
    </O>
</O>

<!-- Create grid of columns -->
<O N="Col_A1" T="Column">
    <P N="x" V="0" />
    <P N="y" V="0" />
    <P N="height" V="3000" />
    <!-- Exported parameters -->
    <P N="top_z" V="height" />
    <P N="volume" V="400 * 400 * height" />
</O>

<O N="Col_A2" T="Column">
    <P N="x" V="5000" />
    <P N="y" V="0" />
    <P N="height" V="3000" />
    <P N="top_z" V="height" />
    <P N="volume" V="400 * 400 * height" />
</O>

<O N="Col_B1" T="Column">
    <P N="x" V="0" />
    <P N="y" V="5000" />
    <P N="height" V="3000" />
    <P N="top_z" V="height" />
    <P N="volume" V="400 * 400 * height" />
</O>

<O N="Col_B2" T="Column">
    <P N="x" V="5000" />
    <P N="y" V="5000" />
    <P N="height" V="3000" />
    <P N="top_z" V="height" />
    <P N="volume" V="400 * 400 * height" />
</O>

<!-- Use exported values -->
<P N="total_volume" V="Col_A1.volume + Col_A2.volume + Col_B1.volume + Col_B2.volume" />

Pattern 2: Instances with Different Parameters

<O N="Beam" T="Project">
    <P N="length" V="1000" Role="Input" />
    <P N="section" V="IPE200" T="Section" Role="Input" />

    <O T="Export">
        <P N="weight" V="length * section.weight_per_length" />
    </O>
</O>

<!-- Different sizes -->
<O N="MainBeam" T="Beam">
    <P N="length" V="8000" />
    <P N="section" V="IPE400" T="Section" />
    <!-- Exported parameters -->
    <P N="weight" V="length * section.weight_per_length" />
</O>

<O N="SecondaryBeam" T="Beam">
    <P N="length" V="4000" />
    <P N="section" V="IPE200" T="Section" />
    <P N="weight" V="length * section.weight_per_length" />
</O>

<!-- Use exported values -->
<P N="total_weight" V="MainBeam.weight + SecondaryBeam.weight * 4" />

Pattern 3: Instance Coordinates from Another Instance

<O N="Foundation" T="Project">
    <P N="x" V="0" Role="Input" />
    <P N="y" V="0" Role="Input" />
    
    <O T="Export">
        <P N="top_elevation" V="100" />
    </O>
</O>

<O N="Column" T="Project">
    <P N="base_x" V="0" Role="Input" />
    <P N="base_y" V="0" Role="Input" />
    <P N="base_z" V="0" Role="Input" />
    <P N="height" V="3000" Role="Input" />

    <O T="Export">
        <P N="top_x" V="base_x" />
        <P N="top_y" V="base_y" />
        <P N="top_z" V="base_z + height" />
    </O>
</O>

<!-- Foundation instance -->
<O N="Found1" T="Foundation">
    <P N="x" V="0" />
    <P N="y" V="0" />
    
    <!-- Exported parameters -->
    <P N="top_elevation" V="100" />
</O>

<!-- Column starts at foundation top -->
<O N="Col1" T="Column">
    <P N="base_x" V="Found1.x" />
    <P N="base_y" V="Found1.y" />
    <P N="base_z" V="Found1.top_elevation" />
    <P N="height" V="3000" />
    
    <!-- Exported parameters -->
    <P N="top_x" V="base_x" />
    <P N="top_y" V="base_y" />
    <P N="top_z" V="base_z + height" />
</O>

<!-- Beam connects column tops -->
<O N="Beam1" T="Line">
    <O T="Point" X="Col1.top_x" Y="Col1.top_y" Z="Col1.top_z" />
    <O T="Point" X="Col2.top_x" Y="Col2.top_y" Z="Col2.top_z" />
</O>

Quick Troubleshooting

Problem: "Can't access parameter from instance"

Check:

Is the parameter inside <O T="Export">?
Does the parameter have Role="Input"?
If neither → it's not accessible from outside

Solution: Move parameter inside Export or add Role="Input"

Problem: "Can't access nested object"

Check:

Is the nested object instance inside <O T="Export">?

Solution: Move the nested object instance inside Export

Problem: "Calculation gives wrong value in instance"

Likely cause: Missing dependency (parameter referenced but not exported)

Solution: See AI-Export.md for the Dependency Rule

Next Steps

You now know:

✅ How to create instances
✅ What gets copied to instances
✅ How to use instance values
✅ Simple nested instances

Quick Reference

Create Instance

<O N="InstanceName" T="ComponentName">
    <P N="input_param" V="value" />
</O>

What Gets Copied

✅ Role="Input" parameters
✅ Everything inside <O T="Export">
❌ Everything else

Access Instance Values

<P N="value" V="InstanceName.exported_param" />
<P N="nested" V="InstanceName.NestedObj.param" />

Export for Accessibility

<O T="Export">
    <P N="accessible_param" V="..." />
    <O N="accessible_obj" T="SomeType" />
</O>

PreviousCore Objects NextExport

Last updated 24 days ago

Good night

hashtagOverview

hashtagWhat are Object Instances?

hashtagAnalogy

hashtagCreating an Instance

hashtagBasic Syntax

hashtagSimple Example

hashtagSimple Example

hashtagWhat Gets Copied to an Instance

hashtag✅ What Gets Copied

hashtag❌ What Does NOT Get Copied

hashtagExample: What Appears

hashtagThe Export Object

hashtagWhy Export Exists

hashtagBasic Export Example

hashtagSetting Instance Parameters

hashtagSet Input Values

hashtagUse Expressions

hashtagReference Other Instances

hashtagNested Instances (Simple)

hashtagSimple Nesting

hashtagThe Exported="1" Attribute

hashtagHow It Works

hashtagCommon Patterns

hashtagPattern 1: Multiple Instances of Same Component

hashtagPattern 2: Instances with Different Parameters

hashtagPattern 3: Instance Coordinates from Another Instance

hashtagQuick Troubleshooting

hashtagProblem: "Can't access parameter from instance"

hashtagProblem: "Can't access nested object"

hashtagProblem: "Calculation gives wrong value in instance"

hashtagNext Steps

hashtagQuick Reference

hashtagCreate Instance

hashtagWhat Gets Copied

hashtagAccess Instance Values

hashtagExport for Accessibility

Overview

What are Object Instances?

Analogy

Creating an Instance

Basic Syntax

Simple Example

Simple Example

What Gets Copied to an Instance

✅ What Gets Copied

❌ What Does NOT Get Copied

Example: What Appears

The Export Object

Why Export Exists

Basic Export Example

Setting Instance Parameters

Set Input Values

Use Expressions

Reference Other Instances

Nested Instances (Simple)

Simple Nesting

The `Exported="1"` Attribute

How It Works

Common Patterns

Pattern 1: Multiple Instances of Same Component

Pattern 2: Instances with Different Parameters

Pattern 3: Instance Coordinates from Another Instance

Quick Troubleshooting

Problem: "Can't access parameter from instance"

Problem: "Can't access nested object"

Problem: "Calculation gives wrong value in instance"

Next Steps

Quick Reference

Create Instance

What Gets Copied

Access Instance Values

Export for Accessibility