feat(smalltrip,problems): new problem package to handle 400 and 500 error codes

This is planned to replace the exceptions package, having a nicer and
more extendable API, based on the RFC 9457 (HTTP problems)

smalltrip/problem/problem.go

@@ -0,0 +1,92 @@
+package problem
+
+import (
+	"encoding/xml"
+	"fmt"
+	"slices"
+)
+
+type Problem struct {
+	Type     string `json:"type,omitempty"     xml:"type,omitempty"`
+	Title    string `json:"title,omitempty"    xml:"title,omitempty"`
+	Status   int    `json:"status,omitempty"   xml:"status,omitempty"`
+	Detail   string `json:"detail,omitempty"   xml:"detail,omitempty"`
+	Instance string `json:"instance,omitempty" xml:"instance,omitempty"`
+
+	XMLName xml.Name `json:"-" xml:"problem"`
+
+	handler func(any) http.Handler `json:"-" xml:"-"`
+}
+
+func New(opts ...Option) Problem {
+	p := Problem{
+		Type:    DefaultType,
+		handler: ProblemHandler,
+	}
+	for _, opt := range opts {
+		opt(&p)
+	}
+	return p
+}
+
+const DefaultType = "about:blank"
+
+func NewStatus(s int, opts ...Option) Problem {
+	return New(slices.Concat([]Option{WithStatus(s)}, opts)...)
+}
+
+func NewDetailed(s int, detail string, opts ...Option) Problem {
+	return New(slices.Concat([]Option{WithStatus(s), WithDetail(detail)}, opts)...)
+}
+
+func (p Problem) StatusCode() int {
+	return p.Status
+}
+
+func WithType(t string) Option {
+	return func(p *Problem) {
+		p.Type = t
+	}
+}
+
+func WithTitle(t string) Option {
+	return func(p *Problem) {
+		p.Title = t
+	}
+}
+
+func WithStatus(s int) Option {
+	return func(p *Problem) {
+		if p.Title == "" {
+			p.Title = http.StatusText(s)
+		}
+
+		p.Status = s
+	}
+}
+
+func WithDetail(d string) Option {
+	return func(p *Problem) {
+		p.Detail = d
+	}
+}
+
+func WithDetailf(f string, args ...any) Option {
+	return func(p *Problem) {
+		p.Detail = fmt.Sprintf(f, args...)
+	}
+}
+
+func WithError(err error) Option {
+	return func(p *Problem) {
+		p.Detail = err.Error()
+	}
+}
+
+func WithInstance(i string) Option {
+	return func(p *Problem) {
+		p.Instance = i
+	}
+}
+
+type Option func(*Problem)