custom-builtin-functions

Policy Engine Extension

The OPA/Rego policy engine supports custom built-in functions written in Go.

Adding Custom Built-ins:

Create Built-in Implementation (e.g., pkg/policies/engine/rego/builtins/myfeature.go):

package builtins

import (
    "github.com/open-policy-agent/opa/ast"
    "github.com/open-policy-agent/opa/topdown"
    "github.com/open-policy-agent/opa/types"
)

const myFuncName = "chainloop.my_function"

func RegisterMyBuiltins() error { return Register(&ast.Builtin{ Name: myFuncName, Description: "Description of what this function does", Decl: types.NewFunction( types.Args(types.Named("input", types.S).Description("this is the input")), types.Named("result", types.S).Description("this is the result"), ), }, myFunctionImpl) }

func myFunctionImpl(bctx topdown.BuiltinContext, operands []*ast.Term, iter func(*ast.Term) error) error { // Extract arguments input, ok := operands[0].Value.(ast.String) if !ok { return fmt.Errorf("input must be a string") }

// Implement logic
result := processInput(string(input))

// Return result
return iter(ast.StringTerm(result))

}

// Autoregisters on package load func init() { if err := RegisterMyBuiltins(); err != nil { panic(fmt.Sprintf("failed to register built-ins: %v", err)) } }


2. **Use in Policies** (`*.rego`):
```rego
package example
import rego.v1

result := {
    "violations": violations,
    "skipped": false
}

violations contains msg if {
    output := chainloop.my_function(input.value)
    output != "expected"
    msg := "Function returned unexpected value"
}

Guidelines:

Use chainloop.* namespace for all custom built-ins
Functions that call third party services should be marked as non-restrictive by adding the NonRestrictiveBuiltin category to the builtin definition
Always implement proper error handling and return meaningful error messages
Use context from BuiltinContext for timeout/cancellation support
Document function signatures and behavior in the Description field and parameter definitions

About this skill

Policy Engine Extension