Added custom printers

cmd/cmsh/fancy.go

@@ -0,0 +1,57 @@
+package main
+
+import (
+	"context"
+	"fmt"
+	"io"
+	"ucl.lmika.dev/repl"
+	"ucl.lmika.dev/ucl"
+)
+
+type FancyType struct {
+	Foo string
+	Bar string
+}
+
+var newFancyDoc = repl.Doc{
+	Brief: "returns something fancy",
+	Detailed: `
+		This will create and return something fancy.
+	`,
+}
+
+func newFancy(ctx context.Context, args ucl.CallArgs) (any, error) {
+	return FancyType{Foo: "is foo", Bar: "is bar"}, nil
+}
+
+func manyFancies(ctx context.Context, args ucl.CallArgs) (any, error) {
+	return []FancyType{
+		{Foo: "foo 1", Bar: "bar 1"},
+		{Foo: "foo 2", Bar: "bar 2"},
+		{Foo: "foo 3", Bar: "bar 3"},
+	}, nil
+}
+
+func displayFancy(w io.Writer, f FancyType, concise bool) error {
+	if concise {
+		_, err := fmt.Fprintf(w, "%s:%s", f.Foo, f.Bar)
+		return err
+	}
+
+	fmt.Fprintf(w, "Foo.. %s\n", f.Foo)
+	fmt.Fprintf(w, "Bar.. %s\n", f.Bar)
+	return nil
+}
+
+func displayFancies(w io.Writer, fs []FancyType, concise bool) error {
+	if concise {
+		_, err := fmt.Fprintf(w, "%d fancies", len(fs))
+		return err
+	}
+
+	fmt.Fprintln(w, "FOO\tBAR")
+	for _, f := range fs {
+		fmt.Fprintf(w, "%v\t%v\n", f.Foo, f.Bar)
+	}
+	return nil
+}

cmd/cmsh/main.go

@@ -33,6 +33,11 @@ func main() {
 			It then terminates.
 		`,
 	})
+	instRepl.SetCommand("new-fancy", newFancy, newFancyDoc)
+	instRepl.SetCommand("many-fancies", manyFancies)
+
+	repl.AddTypePrinter(instRepl, displayFancy)
+	repl.AddTypePrinter(instRepl, displayFancies)
 
 	for {
 		line, err := rl.Readline()

repl/evaldisplay.go

@@ -3,7 +3,10 @@ package repl
 import (
 	"context"
 	"fmt"
+	"io"
 	"os"
+	"reflect"
+	"strings"
 	"ucl.lmika.dev/ucl"
 )
 
@@ -15,31 +18,118 @@ func (r *REPL) EvalAndDisplay(ctx context.Context, expr string) error {
 		return err
 	}
 
-	return displayResult(ctx, r.inst, res)
+	return r.displayResult(ctx, os.Stdout, res, false)
 }
 
-func displayResult(ctx context.Context, inst *ucl.Inst, res any) (err error) {
+func (r *REPL) echoPrinter(ctx context.Context, w io.Writer, args []any) (err error) {
-	switch v := res.(type) {
+	if len(args) == 0 {
-	case NoResults:
+		_, err := fmt.Fprintln(w)
+		return err
+	}
+
+	var line strings.Builder
+	for _, arg := range args {
+		if err := r.displayResult(ctx, &line, arg, len(args) > 1); err != nil {
+			return err
+		}
+	}
+
+	res := line.String()
+	if strings.HasSuffix(res, "\n") {
+		res = res[:len(res)-1]
+	}
+
+	_, err = fmt.Fprintln(w, res)
 	return nil
+}
+
+func (r *REPL) displayResult(ctx context.Context, w io.Writer, res any, concise bool) (err error) {
+	// Check type printers
+	tp, ok := r.typePrinters[reflect.TypeOf(res)]
+	if ok {
+		return tp(w, res, concise)
+	}
+
+	switch v := res.(type) {
 	case nil:
 		if _, err = fmt.Fprintln(os.Stdout, "(nil)"); err != nil {
 			return err
 		}
 	case ucl.Listable:
+		if concise {
+			fmt.Fprintf(w, "[")
 			for i := 0; i < v.Len(); i++ {
-			if err = displayResult(ctx, inst, v.Index(i)); err != nil {
+				if i > 0 {
+					fmt.Fprintf(w, " ")
+				}
+				if err = r.displayResult(ctx, w, v.Index(i), true); err != nil {
 					return err
 				}
 			}
+			fmt.Fprintf(w, "]")
+		} else {
+			for i := 0; i < v.Len(); i++ {
+				if err = r.displayResult(ctx, w, v.Index(i), true); err != nil {
+					return err
+				}
+				fmt.Fprintf(w, "\n")
+			}
+		}
+	case []interface{}:
+		if concise {
+			fmt.Fprintf(w, "[")
+			for i := 0; i < len(v); i++ {
+				if i > 0 {
+					fmt.Fprintf(w, " ")
+				}
+				if err = r.displayResult(ctx, w, v[i], true); err != nil {
+					return err
+				}
+			}
+			fmt.Fprintf(w, "]")
+		} else {
+			// In the off-chance that this is actually a slice of printables
+			vt := reflect.SliceOf(reflect.TypeOf(v[0]))
+			if tp, ok := r.typePrinters[vt]; ok {
+				canDisplay := true
+
+				typeSlice := reflect.MakeSlice(vt, len(v), len(v))
+				for i := 0; i < len(v); i++ {
+					vv := reflect.ValueOf(v[i])
+					if vv.CanConvert(vt.Elem()) {
+						typeSlice.Index(i).Set(vv)
+					} else {
+						canDisplay = false
+						break
+					}
+				}
+
+				if canDisplay {
+					return tp(w, typeSlice.Interface(), concise)
+				}
+			}
+
+			for i := 0; i < len(v); i++ {
+				if err = r.displayResult(ctx, w, v[i], true); err != nil {
+					return err
+				}
+				fmt.Fprintf(w, "\n")
+			}
+		}
 	case ucl.Object:
-		if _, err = fmt.Fprintln(os.Stdout, v.String()); err != nil {
+		if _, err = fmt.Fprint(w, v.String()); err != nil {
 			return err
 		}
+		if !concise {
+			fmt.Fprintln(w)
+		}
 	default:
-		if _, err = fmt.Fprintln(os.Stdout, v); err != nil {
+		if _, err = fmt.Fprint(w, v); err != nil {
 			return err
 		}
+		if !concise {
+			fmt.Fprintln(w)
+		}
 	}
 	return nil
 }

repl/repl.go

@@ -1,6 +1,9 @@
 package repl
 
 import (
+	"io"
+	"reflect"
+	"slices"
 	"ucl.lmika.dev/ucl"
 )
 
@@ -12,15 +15,18 @@ type REPL struct {
 	inst *ucl.Inst
 
 	commandDocs  map[string]Doc
+	typePrinters map[reflect.Type]func(w io.Writer, v any, brief bool) error
 }
 
 func New(opts ...ucl.InstOption) *REPL {
-	inst := ucl.New(opts...)
-
 	r := &REPL{
-		inst:        inst,
 		commandDocs:  make(map[string]Doc),
+		typePrinters: make(map[reflect.Type]func(w io.Writer, v any, brief bool) error),
 	}
+
+	instOpts := append(slices.Clone(opts), ucl.WithCustomEchoPrinter(r.echoPrinter))
+	r.inst = ucl.New(instOpts...)
+
 	r.SetCommand("help", r.helpBuiltin, Doc{
 		Brief: "displays help about a command",
 		Usage: "[command]",
@@ -36,6 +42,8 @@ func New(opts ...ucl.InstOption) *REPL {
 		`,
 	})
 
+	AddTypePrinter(r, func(w io.Writer, t NoResults, concise bool) error { return nil })
+
 	return r
 }
 

repl/typeprinter.go

@@ -0,0 +1,20 @@
+package repl
+
+import (
+	"fmt"
+	"io"
+	"reflect"
+)
+
+func AddTypePrinter[T any](r *REPL, p func(w io.Writer, t T, concise bool) error) {
+	var t T
+
+	tt := reflect.TypeOf(t)
+	r.typePrinters[tt] = func(w io.Writer, v any, concise bool) error {
+		vt, ok := v.(T)
+		if !ok {
+			return fmt.Errorf("cannot convert %v to T", v)
+		}
+		return p(w, vt, concise)
+	}
+}

ucl/builtins.go

@@ -4,10 +4,24 @@ import (
 	"context"
 	"errors"
 	"fmt"
+	"strconv"
 	"strings"
 )
 
-func echoBuiltin(ctx context.Context, args invocationArgs) (object, error) {
+func echoBuiltin(ctx context.Context, args invocationArgs) (Object, error) {
+	echoPrinter := args.inst.echoPrinter
+	if echoPrinter != nil {
+		convertedArgs := make([]interface{}, len(args.args))
+		for i, arg := range args.args {
+			if convArg, ok := toGoValue(arg); ok {
+				convertedArgs[i] = convArg
+			} else {
+				convertedArgs[i] = arg
+			}
+		}
+		return nil, echoPrinter(ctx, args.inst.out, convertedArgs)
+	}
+
 	if len(args.args) == 0 {
 		if _, err := fmt.Fprintln(args.inst.Out()); err != nil {
 			return nil, err
@@ -28,7 +42,7 @@ func echoBuiltin(ctx context.Context, args invocationArgs) (object, error) {
 	return nil, nil
 }
 
-func addBuiltin(ctx context.Context, args invocationArgs) (object, error) {
+func addBuiltin(ctx context.Context, args invocationArgs) (Object, error) {
 	if len(args.args) == 0 {
 		return intObject(0), nil
 	}
@@ -52,7 +66,7 @@ func addBuiltin(ctx context.Context, args invocationArgs) (object, error) {
 	return intObject(n), nil
 }
 
-func subBuiltin(ctx context.Context, args invocationArgs) (object, error) {
+func subBuiltin(ctx context.Context, args invocationArgs) (Object, error) {
 	if len(args.args) == 0 {
 		return intObject(0), nil
 	}
@@ -82,7 +96,7 @@ func subBuiltin(ctx context.Context, args invocationArgs) (object, error) {
 	return intObject(n), nil
 }
 
-func mupBuiltin(ctx context.Context, args invocationArgs) (object, error) {
+func mupBuiltin(ctx context.Context, args invocationArgs) (Object, error) {
 	if len(args.args) == 0 {
 		return intObject(1), nil
 	}
@@ -106,7 +120,7 @@ func mupBuiltin(ctx context.Context, args invocationArgs) (object, error) {
 	return intObject(n), nil
 }
 
-func divBuiltin(ctx context.Context, args invocationArgs) (object, error) {
+func divBuiltin(ctx context.Context, args invocationArgs) (Object, error) {
 	if len(args.args) == 0 {
 		return intObject(1), nil
 	}
@@ -136,7 +150,7 @@ func divBuiltin(ctx context.Context, args invocationArgs) (object, error) {
 	return intObject(n), nil
 }
 
-func modBuiltin(ctx context.Context, args invocationArgs) (object, error) {
+func modBuiltin(ctx context.Context, args invocationArgs) (Object, error) {
 	if len(args.args) == 0 {
 		return intObject(0), nil
 	}
@@ -166,7 +180,7 @@ func modBuiltin(ctx context.Context, args invocationArgs) (object, error) {
 	return intObject(n), nil
 }
 
-func setBuiltin(ctx context.Context, args invocationArgs) (object, error) {
+func setBuiltin(ctx context.Context, args invocationArgs) (Object, error) {
 	if err := args.expectArgn(2); err != nil {
 		return nil, err
 	}
@@ -182,7 +196,7 @@ func setBuiltin(ctx context.Context, args invocationArgs) (object, error) {
 	return newVal, nil
 }
 
-func toUpperBuiltin(ctx context.Context, args invocationArgs) (object, error) {
+func toUpperBuiltin(ctx context.Context, args invocationArgs) (Object, error) {
 	if err := args.expectArgn(1); err != nil {
 		return nil, err
 	}
@@ -193,7 +207,7 @@ func toUpperBuiltin(ctx context.Context, args invocationArgs) (object, error) {
 	return strObject(strings.ToUpper(sarg)), nil
 }
 
-func eqBuiltin(ctx context.Context, args invocationArgs) (object, error) {
+func eqBuiltin(ctx context.Context, args invocationArgs) (Object, error) {
 	if err := args.expectArgn(2); err != nil {
 		return nil, err
 	}
@@ -204,7 +218,7 @@ func eqBuiltin(ctx context.Context, args invocationArgs) (object, error) {
 	return boolObject(objectsEqual(l, r)), nil
 }
 
-func neBuiltin(ctx context.Context, args invocationArgs) (object, error) {
+func neBuiltin(ctx context.Context, args invocationArgs) (Object, error) {
 	if err := args.expectArgn(2); err != nil {
 		return nil, err
 	}
@@ -215,7 +229,7 @@ func neBuiltin(ctx context.Context, args invocationArgs) (object, error) {
 	return boolObject(!objectsEqual(l, r)), nil
 }
 
-func ltBuiltin(ctx context.Context, args invocationArgs) (object, error) {
+func ltBuiltin(ctx context.Context, args invocationArgs) (Object, error) {
 	if err := args.expectArgn(2); err != nil {
 		return nil, err
 	}
@@ -227,7 +241,7 @@ func ltBuiltin(ctx context.Context, args invocationArgs) (object, error) {
 	return boolObject(isLess), nil
 }
 
-func leBuiltin(ctx context.Context, args invocationArgs) (object, error) {
+func leBuiltin(ctx context.Context, args invocationArgs) (Object, error) {
 	if err := args.expectArgn(2); err != nil {
 		return nil, err
 	}
@@ -239,7 +253,7 @@ func leBuiltin(ctx context.Context, args invocationArgs) (object, error) {
 	return boolObject(isLess || objectsEqual(args.args[0], args.args[1])), nil
 }
 
-func gtBuiltin(ctx context.Context, args invocationArgs) (object, error) {
+func gtBuiltin(ctx context.Context, args invocationArgs) (Object, error) {
 	if err := args.expectArgn(2); err != nil {
 		return nil, err
 	}
@@ -251,7 +265,7 @@ func gtBuiltin(ctx context.Context, args invocationArgs) (object, error) {
 	return boolObject(isGreater), nil
 }
 
-func geBuiltin(ctx context.Context, args invocationArgs) (object, error) {
+func geBuiltin(ctx context.Context, args invocationArgs) (Object, error) {
 	if err := args.expectArgn(2); err != nil {
 		return nil, err
 	}
@@ -263,7 +277,7 @@ func geBuiltin(ctx context.Context, args invocationArgs) (object, error) {
 	return boolObject(isGreater || objectsEqual(args.args[0], args.args[1])), nil
 }
 
-func andBuiltin(ctx context.Context, args invocationArgs) (object, error) {
+func andBuiltin(ctx context.Context, args invocationArgs) (Object, error) {
 	if err := args.expectArgn(2); err != nil {
 		return nil, err
 	}
@@ -276,7 +290,7 @@ func andBuiltin(ctx context.Context, args invocationArgs) (object, error) {
 	return args.args[len(args.args)-1], nil
 }
 
-func orBuiltin(ctx context.Context, args invocationArgs) (object, error) {
+func orBuiltin(ctx context.Context, args invocationArgs) (Object, error) {
 	if err := args.expectArgn(2); err != nil {
 		return nil, err
 	}
@@ -289,7 +303,7 @@ func orBuiltin(ctx context.Context, args invocationArgs) (object, error) {
 	return boolObject(false), nil
 }
 
-func notBuiltin(ctx context.Context, args invocationArgs) (object, error) {
+func notBuiltin(ctx context.Context, args invocationArgs) (Object, error) {
 	if err := args.expectArgn(1); err != nil {
 		return nil, err
 	}
@@ -299,7 +313,7 @@ func notBuiltin(ctx context.Context, args invocationArgs) (object, error) {
 
 var errObjectsNotEqual = errors.New("objects not equal")
 
-func objectsEqual(l, r object) bool {
+func objectsEqual(l, r Object) bool {
 	if l == nil || r == nil {
 		return l == nil && r == nil
 	}
@@ -317,8 +331,8 @@ func objectsEqual(l, r object) bool {
 		if rv, ok := r.(boolObject); ok {
 			return lv == rv
 		}
-	case listable:
+	case Listable:
-		rv, ok := r.(listable)
+		rv, ok := r.(Listable)
 		if !ok {
 			return false
 		}
@@ -341,7 +355,7 @@ func objectsEqual(l, r object) bool {
 		if lv.Len() != rv.Len() {
 			return false
 		}
-		if err := lv.Each(func(k string, lkv object) error {
+		if err := lv.Each(func(k string, lkv Object) error {
 			rkv := rv.Value(k)
 			if rkv == nil {
 				return errObjectsNotEqual
@@ -357,7 +371,7 @@ func objectsEqual(l, r object) bool {
 	return false
 }
 
-func objectsLessThan(l, r object) (bool, error) {
+func objectsLessThan(l, r Object) (bool, error) {
 	switch lv := l.(type) {
 	case strObject:
 		if rv, ok := r.(strObject); ok {
@@ -371,7 +385,7 @@ func objectsLessThan(l, r object) (bool, error) {
 	return false, errors.New("objects are not comparable")
 }
 
-func strBuiltin(ctx context.Context, args invocationArgs) (object, error) {
+func strBuiltin(ctx context.Context, args invocationArgs) (Object, error) {
 	if err := args.expectArgn(1); err != nil {
 		return nil, err
 	}
@@ -383,7 +397,7 @@ func strBuiltin(ctx context.Context, args invocationArgs) (object, error) {
 	return strObject(args.args[0].String()), nil
 }
 
-func intBuiltin(ctx context.Context, args invocationArgs) (object, error) {
+func intBuiltin(ctx context.Context, args invocationArgs) (Object, error) {
 	if err := args.expectArgn(1); err != nil {
 		return nil, err
 	}
@@ -411,7 +425,7 @@ func intBuiltin(ctx context.Context, args invocationArgs) (object, error) {
 	return nil, errors.New("cannot convert to int")
 }
 
-func concatBuiltin(ctx context.Context, args invocationArgs) (object, error) {
+func concatBuiltin(ctx context.Context, args invocationArgs) (Object, error) {
 	var sb strings.Builder
 
 	for _, a := range args.args {
@@ -424,7 +438,7 @@ func concatBuiltin(ctx context.Context, args invocationArgs) (object, error) {
 	return strObject(sb.String()), nil
 }
 
-func callBuiltin(ctx context.Context, args invocationArgs) (object, error) {
+func callBuiltin(ctx context.Context, args invocationArgs) (Object, error) {
 	if err := args.expectArgn(1); err != nil {
 		return nil, err
 	}
@@ -437,7 +451,7 @@ func callBuiltin(ctx context.Context, args invocationArgs) (object, error) {
 	return inv.invoke(ctx, args.shift(1))
 }
 
-func lenBuiltin(ctx context.Context, args invocationArgs) (object, error) {
+func lenBuiltin(ctx context.Context, args invocationArgs) (Object, error) {
 	if err := args.expectArgn(1); err != nil {
 		return nil, err
 	}
@@ -445,7 +459,7 @@ func lenBuiltin(ctx context.Context, args invocationArgs) (object, error) {
 	switch v := args.args[0].(type) {
 	case strObject:
 		return intObject(len(string(v))), nil
-	case listable:
+	case Listable:
 		return intObject(v.Len()), nil
 	case hashable:
 		return intObject(v.Len()), nil
@@ -454,9 +468,9 @@ func lenBuiltin(ctx context.Context, args invocationArgs) (object, error) {
 	return intObject(0), nil
 }
 
-func indexLookup(ctx context.Context, obj, elem object) (object, error) {
+func indexLookup(ctx context.Context, obj, elem Object) (Object, error) {
 	switch v := obj.(type) {
-	case listable:
+	case Listable:
 		intIdx, ok := elem.(intObject)
 		if !ok {
 			return nil, nil
@@ -475,7 +489,7 @@ func indexLookup(ctx context.Context, obj, elem object) (object, error) {
 	return nil, nil
 }
 
-func indexBuiltin(ctx context.Context, args invocationArgs) (object, error) {
+func indexBuiltin(ctx context.Context, args invocationArgs) (Object, error) {
 	if err := args.expectArgn(1); err != nil {
 		return nil, err
 	}
@@ -492,7 +506,28 @@ func indexBuiltin(ctx context.Context, args invocationArgs) (object, error) {
 	return val, nil
 }
 
-func mapBuiltin(ctx context.Context, args invocationArgs) (object, error) {
+func keysBuiltin(ctx context.Context, args invocationArgs) (Object, error) {
+	if err := args.expectArgn(1); err != nil {
+		return nil, err
+	}
+
+	val := args.args[0]
+	switch v := val.(type) {
+	case hashable:
+		keys := make(listObject, 0, v.Len())
+		if err := v.Each(func(k string, _ Object) error {
+			keys = append(keys, strObject(k))
+			return nil
+		}); err != nil {
+			return nil, err
+		}
+		return keys, nil
+	}
+
+	return nil, nil
+}
+
+func mapBuiltin(ctx context.Context, args invocationArgs) (Object, error) {
 	if err := args.expectArgn(2); err != nil {
 		return nil, err
 	}
@@ -503,12 +538,12 @@ func mapBuiltin(ctx context.Context, args invocationArgs) (object, error) {
 	}
 
 	switch t := args.args[0].(type) {
-	case listable:
+	case Listable:
 		l := t.Len()
 		newList := listObject{}
 		for i := 0; i < l; i++ {
 			v := t.Index(i)
-			m, err := inv.invoke(ctx, args.fork([]object{v}))
+			m, err := inv.invoke(ctx, args.fork([]Object{v}))
 			if err != nil {
 				return nil, err
 			}
@@ -525,7 +560,7 @@ func firstBuiltin(ctx context.Context, args invocationArgs) (object, error) {
 	}
 
 	switch t := args.args[0].(type) {
-	case listable:
+	case Listable:
 		if t.Len() == 0 {
 			return nil, nil
 		}
@@ -561,7 +596,7 @@ func (s seqObject) Len() int {
 	return l
 }
 
-func (s seqObject) Index(i int) object {
+func (s seqObject) Index(i int) Object {
 	l := s.Len()
 	if i < 0 || i > l {
 		return nil
@@ -572,7 +607,7 @@ func (s seqObject) Index(i int) object {
 	return intObject(s.from + i)
 }
 
-func seqBuiltin(ctx context.Context, args invocationArgs) (object, error) {
+func seqBuiltin(ctx context.Context, args invocationArgs) (Object, error) {
 	inclusive := false
 	if inc, ok := args.kwargs["inc"]; ok {
 		inclusive = (inc.Len() == 0) || inc.Truthy()
@@ -601,7 +636,7 @@ func seqBuiltin(ctx context.Context, args invocationArgs) (object, error) {
 	}
 }
 
-func ifBuiltin(ctx context.Context, args macroArgs) (object, error) {
+func ifBuiltin(ctx context.Context, args macroArgs) (Object, error) {
 	if args.nargs() < 2 {
 		return nil, errors.New("need at least 2 arguments")
 	}
@@ -641,7 +676,7 @@ func ifBuiltin(ctx context.Context, args macroArgs) (object, error) {
 
 func foreachBuiltin(ctx context.Context, args macroArgs) (object, error) {
 	var (
-		items    object
+		items    Object
 		blockIdx int
 		err      error
 	)
@@ -664,16 +699,16 @@ func foreachBuiltin(ctx context.Context, args macroArgs) (object, error) {
 	}
 
 	var (
-		last     object
+		last     Object
 		breakErr errBreak
 	)
 
 	switch t := items.(type) {
-	case listable:
+	case Listable:
 		l := t.Len()
 		for i := 0; i < l; i++ {
 			v := t.Index(i)
-			last, err = args.evalBlock(ctx, blockIdx, []object{v}, true) // TO INCLUDE: the index
+			last, err = args.evalBlock(ctx, blockIdx, []Object{v}, true) // TO INCLUDE: the index
 			if err != nil {
 				if errors.As(err, &breakErr) {
 					if !breakErr.isCont {
@@ -685,8 +720,8 @@ func foreachBuiltin(ctx context.Context, args macroArgs) (object, error) {
 			}
 		}
 	case hashable:
-		err := t.Each(func(k string, v object) error {
+		err := t.Each(func(k string, v Object) error {
-			last, err = args.evalBlock(ctx, blockIdx, []object{strObject(k), v}, true)
+			last, err = args.evalBlock(ctx, blockIdx, []Object{strObject(k), v}, true)
 			return err
 		})
 		if errors.As(err, &breakErr) {
@@ -701,25 +736,25 @@ func foreachBuiltin(ctx context.Context, args macroArgs) (object, error) {
 	return last, nil
 }
 
-func breakBuiltin(ctx context.Context, args invocationArgs) (object, error) {
+func breakBuiltin(ctx context.Context, args invocationArgs) (Object, error) {
 	if len(args.args) < 1 {
 		return nil, errBreak{}
 	}
 	return nil, errBreak{ret: args.args[0]}
 }
 
-func continueBuiltin(ctx context.Context, args invocationArgs) (object, error) {
+func continueBuiltin(ctx context.Context, args invocationArgs) (Object, error) {
 	return nil, errBreak{isCont: true}
 }
 
-func returnBuiltin(ctx context.Context, args invocationArgs) (object, error) {
+func returnBuiltin(ctx context.Context, args invocationArgs) (Object, error) {
 	if len(args.args) < 1 {
 		return nil, errReturn{}
 	}
 	return nil, errReturn{ret: args.args[0]}
 }
 
-func procBuiltin(ctx context.Context, args macroArgs) (object, error) {
+func procBuiltin(ctx context.Context, args macroArgs) (Object, error) {
 	if args.nargs() < 1 {
 		return nil, errors.New("need at least one arguments")
 	}
@@ -763,7 +798,7 @@ func (b procObject) Truthy() bool {
 	return true
 }
 
-func (b procObject) invoke(ctx context.Context, args invocationArgs) (object, error) {
+func (b procObject) invoke(ctx context.Context, args invocationArgs) (Object, error) {
 	newEc := b.ec.fork()
 
 	for i, name := range b.block.Names {

ucl/inst.go

@@ -11,6 +11,7 @@ import (
 type Inst struct {
 	out                   io.Writer
 	missingBuiltinHandler MissingBuiltinHandler
+	echoPrinter           EchoPrinter
 
 	rootEC *evalCtx
 }
@@ -37,6 +38,14 @@ func WithModule(module Module) InstOption {
 	}
 }
 
+type EchoPrinter func(ctx context.Context, w io.Writer, args []any) error
+
+func WithCustomEchoPrinter(echoPrinter EchoPrinter) InstOption {
+	return func(i *Inst) {
+		i.echoPrinter = echoPrinter
+	}
+}
+
 type Module struct {
 	Name     string
 	Builtins map[string]BuiltinHandler