classad_expr_demo

ClassAd and Expr Fields Demo

This example demonstrates how to use *classad.ClassAd and *classad.Expr fields in Go structs when marshaling and unmarshaling ClassAds.

Features Demonstrated

1. *classad.ClassAd Fields: Store nested ClassAds without defining intermediate struct types
2. *classad.Expr Fields: Preserve unevaluated expressions for later evaluation in different contexts
3. Nil Handling: How nil *ClassAd and *Expr fields are marshaled

Running the Example

go run main.go

Example 1: *classad.ClassAd Fields

Shows how to embed arbitrary ClassAds within a struct:

type ServiceConfig struct {
    Name     string           `classad:"Name"`
    Database *classad.ClassAd `classad:"Database"`
    Cache    *classad.ClassAd `classad:"Cache"`
}

This is useful when:

• You don't want to define a separate struct for nested configuration
• The nested structure varies dynamically
• You need to preserve the exact ClassAd structure

Example 2: *classad.Expr Fields

Shows how to preserve unevaluated expressions:

type JobTemplate struct {
    BaseCPUs       int           `classad:"BaseCPUs"`
    ComputedCPUs   *classad.Expr `classad:"ComputedCPUs"`
}

Key benefits:

• Expressions are NOT evaluated during marshal/unmarshal
• Can evaluate later with different contexts
• Perfect for templates and formulas

// Create expression
cpuExpr, _ := classad.ParseExpr("BaseCPUs * ScaleFactor")

// Marshal preserves the formula
template := JobTemplate{BaseCPUs: 2, ComputedCPUs: cpuExpr}
classadStr, _ := classad.Marshal(template)
// Result: [BaseCPUs = 2; ComputedCPUs = BaseCPUs * ScaleFactor]

// Evaluate with context
context := classad.New()
context.InsertAttr("ScaleFactor", 4)
result := restored.ComputedCPUs.Eval(context)  // 2 * 4 = 8

Example 3: Nil Fields

Shows how nil *ClassAd and *Expr fields are handled:

type OptionalConfig struct {
    Name     string           `classad:"Name"`
    Database *classad.ClassAd `classad:"Database,omitempty"`
    Formula  *classad.Expr    `classad:"Formula,omitempty"`
}

// Nil fields are marshaled as undefined (or omitted with omitempty)
config := OptionalConfig{Name: "test", Database: nil, Formula: nil}

Use Cases

Dynamic Configuration

type AppConfig struct {
    Name     string
    Settings *classad.ClassAd  // Can vary per deployment
}

Job Templates

type JobTemplate struct {
    BaseCPUs   int
    BaseMemory int
    Formula    *classad.Expr  // Evaluated per job
}

Multi-Environment Configs

type DeployConfig struct {
    App         string
    Development *classad.ClassAd
    Production  *classad.ClassAd
}

See Also

• MARSHALING.md - Complete marshaling guide
• MARSHALING_QUICKREF.md - Quick reference
• struct_demo - Basic struct marshaling examples