Add contextual information to mapper errors.

_examples/server/main.go

@@ -8,12 +8,11 @@ import (
 	"os"
 	"strings"
 
-	"golang.org/x/crypto/ssh/terminal"
-
 	"github.com/chzyer/readline"
 	"github.com/gliderlabs/ssh"
 	"github.com/google/shlex"
 	"github.com/kr/pty"
+	"golang.org/x/crypto/ssh/terminal"
 
 	"github.com/alecthomas/colour"
 	"github.com/alecthomas/kong"

model.go

@@ -253,10 +253,11 @@ 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 {
 	err := v.Mapper.Decode(&DecodeContext{Value: v, Scan: scan}, target)
-	if err == nil {
-		v.Set = true
+	if err != nil {
+		return fmt.Errorf("%s: %s", v.Summary(), err)
 	}
-	return err
+	v.Set = true
+	return nil
 }
 
 // Apply value to field.