feat(binding): add CustomDecimal type for parsing decimal numbers wit…

…h leading dot

This commit adds support for parsing decimal numbers that start with a dot
(e.g. '.1') in query parameters and form data. It implements the
BindUnmarshaler interface to handle this special case.

Fixes #4089

binding/decimal.go

@@ -0,0 +1,29 @@
+package binding
+
+import (
+	"github.com/shopspring/decimal"
+	"strings"
+)
+
+// CustomDecimal represents a decimal number that can be bound from form values.
+// It supports values with leading dots (e.g. ".1" is parsed as "0.1").
+type CustomDecimal struct {
+	decimal.Decimal
+}
+
+// UnmarshalParam implements the binding.BindUnmarshaler interface.
+// It converts form values to decimal.Decimal, with special handling for
+// values that start with a dot (e.g. ".1" becomes "0.1").
+func (cd *CustomDecimal) UnmarshalParam(val string) error {
+	if strings.HasPrefix(val, ".") {
+		val = "0" + val
+	}
+
+	dec, err := decimal.NewFromString(val)
+	if err != nil {
+		return err
+	}
+
+	cd.Decimal = dec
+	return nil
+}

binding/decimal_test.go

@@ -0,0 +1,59 @@
+package binding
+
+import (
+	"github.com/stretchr/testify/assert"
+	"testing"
+)
+
+func TestCustomDecimalUnmarshalParam(t *testing.T) {
+	tests := []struct {
+		name    string
+		input   string
+		want    string
+		wantErr bool
+	}{
+		{
+			name:    "leading dot",
+			input:   ".1",
+			want:    "0.1",
+			wantErr: false,
+		},
+		{
+			name:    "invalid decimal",
+			input:   "abc",
+			wantErr: true,
+		},
+		{
+			name:    "empty string",
+			input:   "",
+			wantErr: true,
+		},
+		{
+			name:    "leading dot with multiple digits",
+			input:   ".123",
+			want:    "0.123",
+			wantErr: false,
+		},
+		{
+			name:    "normal decimal",
+			input:   "1.23",
+			want:    "1.23",
+			wantErr: false,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			var cd CustomDecimal
+			err := cd.UnmarshalParam(tt.input)
+
+			if tt.wantErr {
+				assert.Error(t, err)
+				return
+			}
+
+			assert.NoError(t, err)
+			assert.Equal(t, tt.want, cd.String())
+		})
+	}
+}

examples/custom-decimal/main.go

@@ -0,0 +1,31 @@
+package main
+
+import (
+	"github.com/gin-gonic/gin"
+	"github.com/gin-gonic/gin/binding"
+	"net/http"
+)
+
+type QueryParams struct {
+	Amount binding.CustomDecimal `form:"amount"`
+}
+
+func main() {
+	r := gin.Default()
+
+	r.GET("/amount", func(c *gin.Context) {
+		var params QueryParams
+		if err := c.BindQuery(&params); err != nil {
+			c.JSON(http.StatusBadRequest, gin.H{
+				"error": err.Error(),
+			})
+			return
+		}
+
+		c.JSON(http.StatusOK, gin.H{
+			"amount": params.Amount.String(),
+		})
+	})
+
+	r.Run(":8080")
+}