feat: better convert usage

internal/cli/conversion.go

@@ -21,7 +21,7 @@ type forwardCodec[T, U any] struct {
 func (c forwardCodec[T, U]) Run(r Repr) (Repr, error) {
 	t, ok := r.Data().(T)
 	if !ok {
-		return nil, fmt.Errorf("could not parse '%v' as '%s'", t, c.outType)
+		return nil, fmt.Errorf("could not parse '%v' as '%s'", t, c.inType)
 	}
 
 	u, err := c.codec.Encode(t)
@@ -29,7 +29,7 @@ func (c forwardCodec[T, U]) Run(r Repr) (Repr, error) {
 		return nil, err
 	}
 
-	return NewRepr(c.inType, u), nil
+	return NewRepr(c.outType, u), nil
 }
 
 func (c forwardCodec[T, U]) InType() string { return c.inType }
@@ -44,7 +44,7 @@ type backwardCodec[T, U any] struct {
 func (c backwardCodec[T, U]) Run(r Repr) (Repr, error) {
 	u, ok := r.Data().(U)
 	if !ok {
-		return nil, fmt.Errorf("could not parse '%v' as '%s'", r, c.inType)
+		return nil, fmt.Errorf("could not parse '%v' as '%s'", r, c.outType)
 	}
 
 	t, err := c.codec.Decode(u)
@@ -52,7 +52,7 @@ func (c backwardCodec[T, U]) Run(r Repr) (Repr, error) {
 		return nil, err
 	}
 
-	return NewRepr(c.outType, t), nil
+	return NewRepr(c.inType, t), nil
 }
 
 func (c backwardCodec[T, U]) InType() string { return c.outType }

internal/cli/marshaler.go

@@ -2,6 +2,7 @@ package cli
 
 import (
 	"fmt"
+	"reflect"
 
 	"git.maximhutz.com/max/lambda/pkg/codec"
 )
@@ -29,7 +30,9 @@ func (c convertedMarshaler[T]) Decode(s string) (Repr, error) {
 func (c convertedMarshaler[T]) Encode(r Repr) (string, error) {
 	t, ok := r.Data().(T)
 	if !ok {
-		return "", fmt.Errorf("could not parse '%v' as 'string'", t)
+		dataType := reflect.TypeOf(r.Data())
+		allowedType := reflect.TypeFor[T]()
+		return "", fmt.Errorf("marshaler for '%s' cannot parse '%s'", allowedType, dataType)
 	}
 
 	return c.codec.Encode(t)