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 ) ) } } Use in Policies ( .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