feat: better convert usage

cmd/lambda/lambda_convert.go

@@ -25,11 +25,14 @@ func LambdaConvert() *cobra.Command {
 	var inputReprFlag, outputReprFlag string
 
 	cmd := &cobra.Command{
-		Use:          "convert <input> <output>",
+		Use:          "convert <input-file> <output-file>",
 		Short:        "Convert between lambda calculus representations",
-		Args:         cobra.ExactArgs(2),
 		SilenceUsage: true,
-		RunE: func(_ *cobra.Command, args []string) error {
+		RunE: func(cmd *cobra.Command, args []string) error {
+			if len(args) != 2 {
+				return cmd.Help()
+			}
+
 			var err error
 			inputPath, outputPath := args[0], args[1]
 
@@ -71,7 +74,7 @@ func LambdaConvert() *cobra.Command {
 			// Marshal output.
 			output, err := r.Marshal(result)
 			if err != nil {
-				return fmt.Errorf("marshaling output: %w", err)
+				return fmt.Errorf("unmarshaling output: %w", err)
 			}
 
 			// Write output file.

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)