add expr lang processor

go.mod

@@ -4,6 +4,7 @@ go 1.25.3
 
 require (
 	github.com/eclipse/paho.mqtt.golang v1.5.1
+	github.com/expr-lang/expr v1.17.6
 	github.com/jwetzell/osc-go v0.1.0
 	github.com/jwetzell/psn-go v0.2.0
 	github.com/urfave/cli/v3 v3.6.1

go.sum

@@ -2,6 +2,8 @@ github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c
 github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
 github.com/eclipse/paho.mqtt.golang v1.5.1 h1:/VSOv3oDLlpqR2Epjn1Q7b2bSTplJIeV2ISgCl2W7nE=
 github.com/eclipse/paho.mqtt.golang v1.5.1/go.mod h1:1/yJCneuyOoCOzKSsOTUc0AJfpsItBGWvYpBLimhArU=
+github.com/expr-lang/expr v1.17.6 h1:1h6i8ONk9cexhDmowO/A64VPxHScu7qfSl2k8OlINec=
+github.com/expr-lang/expr v1.17.6/go.mod h1:8/vRC7+7HBzESEqt5kKpYXxrxkr31SaO8r40VO/1IT4=
 github.com/google/go-cmp v0.5.9 h1:O2Tfq5qg4qc4AmwVlvv0oLiVAGB7enBSJ2x2DqQFi38=
 github.com/google/go-cmp v0.5.9/go.mod h1:17dUlkBOakJ0+DkrSSNjCkIjxS6bF9zb3elmeNGIjoY=
 github.com/gorilla/websocket v1.5.3 h1:saDtZ6Pbx/0u+bgYQ3q96pZgCzfhKXGPqt7kZ72aNNg=

internal/processing/debug-expr.go

@@ -0,0 +1,57 @@
+package processing
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/expr-lang/expr"
+	"github.com/expr-lang/expr/vm"
+)
+
+// NOTE(jwetzell): see language definition https://expr-lang.org/docs/language-definition
+type DebugExpr struct {
+	config  ProcessorConfig
+	Program *vm.Program
+}
+
+func (dl *DebugExpr) Process(ctx context.Context, payload any) (any, error) {
+
+	output, err := expr.Run(dl.Program, payload)
+	if err != nil {
+		return nil, err
+	}
+
+	return output, nil
+}
+
+func (dl *DebugExpr) Type() string {
+	return dl.config.Type
+}
+
+func init() {
+	RegisterProcessor(ProcessorRegistration{
+		Type: "debug.expr",
+		New: func(config ProcessorConfig) (Processor, error) {
+			params := config.Params
+
+			expression, ok := params["expression"]
+
+			if !ok {
+				return nil, fmt.Errorf("debug.expr requires an expression parameter")
+			}
+
+			expressionString, ok := expression.(string)
+
+			if !ok {
+				return nil, fmt.Errorf("debug.expr url must be a string")
+			}
+
+			program, err := expr.Compile(expressionString)
+			if err != nil {
+				return nil, err
+			}
+
+			return &DebugExpr{config: config, Program: program}, nil
+		},
+	})
+}