Make invalid value error messages more useful.

_examples/shell/main.go

@@ -2,7 +2,6 @@ package main
 
 import (
 	"fmt"
-
 	"github.com/alecthomas/kong"
 )
 

kong_test.go

@@ -715,7 +715,7 @@ func TestNumericParamErrors(t *testing.T) {
 	}
 	parser := mustNew(t, &cli)
 	_, err := parser.Parse([]string{"--name", "-10"})
-	require.EqualError(t, err, `expected string value but got "-10" (short flag)`)
+	require.EqualError(t, err, `--name: expected string value but got "-10" (short flag)`)
 }
 
 func TestDefaultValueIsHyphen(t *testing.T) {

mapper.go

@@ -75,7 +75,7 @@ type BoolMapper interface {
 // A MapperFunc is a single function that complies with the Mapper interface.
 type MapperFunc func(ctx *DecodeContext, target reflect.Value) error
 
-func (m MapperFunc) Decode(ctx *DecodeContext, target reflect.Value) error { //nolint: golint
+func (m MapperFunc) Decode(ctx *DecodeContext, target reflect.Value) error { // nolint: golint
 	return m(ctx, target)
 }
 
@@ -224,9 +224,10 @@ func (boolMapper) IsBool() bool { return true }
 
 func durationDecoder() MapperFunc {
 	return func(ctx *DecodeContext, target reflect.Value) error {
-		r, err := time.ParseDuration(ctx.Scan.PopValue("duration"))
+		value := ctx.Scan.PopValue("duration")
+		r, err := time.ParseDuration(value)
 		if err != nil {
-			return err
+			return fmt.Errorf("expected duration but got %q: %s", value, err)
 		}
 		target.Set(reflect.ValueOf(r))
 		return nil
@@ -235,11 +236,12 @@ func durationDecoder() MapperFunc {
 
 func timeDecoder() MapperFunc {
 	return func(ctx *DecodeContext, target reflect.Value) error {
-		fmt := time.RFC3339
+		format := time.RFC3339
 		if ctx.Value.Format != "" {
-			fmt = ctx.Value.Format
+			format = ctx.Value.Format
 		}
-		t, err := time.Parse(fmt, ctx.Scan.PopValue("time"))
+		value := ctx.Scan.PopValue("time")
+		t, err := time.Parse(format, value)
 		if err != nil {
 			return err
 		}
@@ -253,7 +255,7 @@ func intDecoder(bits int) MapperFunc {
 		value := ctx.Scan.PopValue("int")
 		n, err := strconv.ParseInt(value, 10, bits)
 		if err != nil {
-			return fmt.Errorf("invalid int %q", value)
+			return fmt.Errorf("expected int but got %q", value)
 		}
 		target.SetInt(n)
 		return nil
@@ -265,7 +267,7 @@ func uintDecoder(bits int) MapperFunc {
 		value := ctx.Scan.PopValue("uint")
 		n, err := strconv.ParseUint(value, 10, bits)
 		if err != nil {
-			return fmt.Errorf("invalid uint %q", value)
+			return fmt.Errorf("expected unsigned int but got %q", value)
 		}
 		target.SetUint(n)
 		return nil
@@ -277,7 +279,7 @@ func floatDecoder(bits int) MapperFunc {
 		value := ctx.Scan.PopValue("float")
 		n, err := strconv.ParseFloat(value, bits)
 		if err != nil {
-			return fmt.Errorf("invalid float %q", value)
+			return fmt.Errorf("expected float but got %q", value)
 		}
 		target.SetFloat(n)
 		return nil

model.go

@@ -226,6 +226,21 @@ func (v *Value) EnumMap() map[string]bool {
 	return out
 }
 
+// ShortSummary returns a human-readable summary of the value, not including any placeholders/defaults.
+func (v *Value) ShortSummary() string {
+	if v.Flag != nil {
+		return fmt.Sprintf("--%s", v.Name)
+	}
+	argText := "<" + v.Name + ">"
+	if v.IsCumulative() {
+		argText += " ..."
+	}
+	if !v.Required {
+		argText = "[" + argText + "]"
+	}
+	return argText
+}
+
 // Summary returns a human-readable summary of the value.
 func (v *Value) Summary() string {
 	if v.Flag != nil {
@@ -268,20 +283,20 @@ func (v *Value) IsBool() bool {
 }
 
 // Parse tokens into value, parse, and validate, but do not write to the field.
-func (v *Value) Parse(scan *Scanner, target reflect.Value) error {
+func (v *Value) Parse(scan *Scanner, target reflect.Value) (err error) {
 	defer func() {
-		if err := recover(); err != nil {
+		if rerr := recover(); rerr != nil {
-			switch err := err.(type) {
+			switch rerr := rerr.(type) {
 			case Error:
-				panic(err)
+				err = fmt.Errorf("%s: %s", v.ShortSummary(), rerr)
 			default:
-				panic(fmt.Sprintf("mapper %T failed to apply to %s: %s", v.Mapper, v.Summary(), err))
+				panic(fmt.Sprintf("mapper %T failed to apply to %s: %s", v.Mapper, v.Summary(), rerr))
 			}
 		}
 	}()
-	err := v.Mapper.Decode(&DecodeContext{Value: v, Scan: scan}, target)
+	err = v.Mapper.Decode(&DecodeContext{Value: v, Scan: scan}, target)
 	if err != nil {
-		return fmt.Errorf("%s: %s", v.Summary(), err)
+		return fmt.Errorf("%s: %s", v.ShortSummary(), err)
 	}
 	v.Set = true
 	return nil