add generic context utility (#41)

contextx/contextx.go

@@ -0,0 +1,54 @@
+package contextx
+
+import (
+	"context"
+)
+
+// key is a unique type that we can use as a key in a context
+type key[T any] struct{}
+
+// With returns a copy of parent that contains the given value which can be retrieved by calling From with the resulting context
+// The function uses a generic key type to ensure that the stored value is type-safe and can be uniquely identified and retrieved without
+// risk of key collisions
+func With[T any](ctx context.Context, v T) context.Context {
+	return context.WithValue(ctx, key[T]{}, v)
+}
+
+// From returns the value associated with the wanted type from the context
+// It performs a type assertion to convert the value to the desired type T
+// If the type assertion is successful, it returns the value and true
+// If the type assertion fails, it returns the zero value of type T and false
+func From[T any](ctx context.Context) (T, bool) {
+	v, ok := ctx.Value(key[T]{}).(T)
+
+	return v, ok
+}
+
+// MustFrom is similar to from, except that it panics if the type assertion fails / the value is not in the context
+func MustFrom[T any](ctx context.Context) T {
+	return ctx.Value(key[T]{}).(T)
+}
+
+// FromOr returns the value associated with the wanted type or the given default value if the type is not found
+// This function is useful when you want to ensure that a value is always returned from the context, even if the
+// context does not contain a value of the desired type. By providing a default value, you can avoid handling
+// the case where the value is missing and ensure that your code has a fallback value to use
+func FromOr[T any](ctx context.Context, def T) T {
+	v, ok := From[T](ctx)
+	if !ok {
+		return def
+	}
+
+	return v
+}
+
+// FromOrFunc returns the value associated with the wanted type or the result of the given function if the type is not found
+// This function is useful when the default value is expensive to compute or when the default value depends on some runtime conditions
+func FromOrFunc[T any](ctx context.Context, f func() T) T {
+	v, ok := From[T](ctx)
+	if !ok {
+		return f()
+	}
+
+	return v
+}

contextx/contextx_test.go

@@ -0,0 +1,102 @@
+package contextx
+
+import (
+	"context"
+	"reflect"
+	"testing"
+)
+
+func TestNormalOperation(t *testing.T) {
+	ctx := context.Background()
+	ctx = With(ctx, 10)
+
+	if MustFrom[int](ctx) != 10 {
+		t.FailNow()
+	}
+
+	if _, ok := From[float64](ctx); ok {
+		t.FailNow()
+	}
+}
+
+func TestIsolatedFromExplicitTypeReflection(t *testing.T) {
+	ctx := context.Background()
+
+	ctx = With(ctx, 10)
+
+	ctx = context.WithValue(ctx, reflect.TypeOf(20), 20)
+
+	if MustFrom[int](ctx) != 10 {
+		t.FailNow()
+	}
+}
+
+func TestPanicIfNoValue(t *testing.T) {
+	defer func() {
+		if recover() == nil {
+			t.FailNow()
+		}
+	}()
+
+	MustFrom[int](context.Background())
+}
+
+type x interface {
+	a()
+}
+
+type y struct{ v int }
+
+func (y) a() {}
+
+type z struct{ f func() }
+
+func (z z) a() { z.f() }
+
+func TestShouldWorkOnInterface(t *testing.T) {
+	var a x = y{10}
+
+	ctx := context.Background()
+	ctx = With(ctx, a)
+
+	b := MustFrom[x](ctx)
+	if b.(y).v != 10 {
+		t.FailNow()
+	}
+
+	r := ""
+	a = z{func() { r = "hello" }}
+
+	ctx = With(ctx, a)
+
+	MustFrom[x](ctx).a()
+
+	if r != "hello" {
+		t.FailNow()
+	}
+}
+func TestFromOr(t *testing.T) {
+	ctx := context.Background()
+	ctx = With(ctx, 10)
+
+	if FromOr(ctx, 20) != 10 {
+		t.FailNow()
+	}
+
+	if FromOr(context.Background(), 20) != 20 {
+		t.FailNow()
+	}
+}
+
+func TestFromOrFunc(t *testing.T) {
+	ctx := context.Background()
+	ctx = With(ctx, 10)
+
+	if FromOrFunc(ctx, func() int { return 20 }) != 10 {
+		t.FailNow()
+	}
+
+	if FromOrFunc(context.Background(), func() int { return 20 }) != 20 {
+		t.FailNow()
+	}
+}

contextx/doc.go

@@ -0,0 +1,8 @@
+// Package contextx is a helper package for managing context values
+// Most **request-scoped data** is a singleton per request
+// That is, it doesn't make sense  for a request to carry around multiple loggers, users, traces
+// you want to carry the _same one_ with you from function call to function call
+// the way we've handled this historically is a separate context key per type you want to carry in the struct
+// but with generics, instead of having to make a new zero-sized type for every struct
+// we can just make a single generic type and use it for everything which is what this helper package is intended to do
+package contextx