add support to convert testcase variables from one type to an another

let/As.go

@@ -0,0 +1,25 @@
+package let
+
+import (
+	"fmt"
+	"github.com/adamluzsi/testcase"
+	"reflect"
+)
+
+func As[To, From any](Var testcase.Var[From]) testcase.Var[To] {
+	asID++
+	fromType := reflect.TypeOf((*From)(nil)).Elem()
+	toType := reflect.TypeOf((*To)(nil)).Elem()
+	if !fromType.ConvertibleTo(toType) {
+		panic(fmt.Sprintf("you can't have %s as %s", fromType.String(), toType.String()))
+	}
+	return testcase.Var[To]{
+		ID: fmt.Sprintf("%s AS %T #%d", Var.ID, *new(To), asID),
+		Init: func(t *testcase.T) To {
+			var rFrom = reflect.ValueOf(Var.Get(t))
+			return rFrom.Convert(toType).Interface().(To)
+		},
+	}
+}
+
+var asID int // adds extra safety that there won't be a name collision between two variables

let/As_test.go

@@ -0,0 +1,49 @@
+package let_test
+
+import (
+	"github.com/adamluzsi/testcase"
+	"github.com/adamluzsi/testcase/assert"
+	"github.com/adamluzsi/testcase/let"
+	"github.com/adamluzsi/testcase/sandbox"
+	"testing"
+	"time"
+)
+
+func TestAs(t *testing.T) {
+	t.Run("primitive type", func(t *testing.T) {
+		type MyString string
+
+		s := testcase.NewSpec(t)
+		v1 := let.String(s)
+		v2 := let.As[MyString](v1)
+
+		s.Test("", func(t *testcase.T) {
+			t.Must.Equal(MyString(v1.Get(t)), v2.Get(t))
+		})
+	})
+
+	t.Run("interface type", func(t *testing.T) {
+		type TimeAfterer interface {
+			After(u time.Time) bool
+		}
+
+		s := testcase.NewSpec(t)
+		v1 := let.Time(s)
+		v2 := let.As[TimeAfterer](v1)
+
+		s.Test("", func(t *testcase.T) {
+			t.Must.Equal(TimeAfterer(v1.Get(t)), v2.Get(t))
+		})
+	})
+
+	t.Run("panics on incorrect conversation", func(t *testing.T) {
+		ro := sandbox.Run(func() {
+			s := testcase.NewSpec(t)
+			v1 := let.Time(s)
+			_ = let.As[string](v1)
+		})
+		assert.False(t, ro.OK)
+		assert.False(t, ro.Goexit)
+		assert.NotNil(t, ro.PanicValue)
+	})
+}

let/example_test.go

@@ -194,3 +194,14 @@ func ExampleWith_testcaseTFunc() {
 		t.Log(v.Get(t))
 	})
 }
+
+func ExampleAs() {
+	s := testcase.NewSpec((testing.TB)(nil))
+
+	type MyString string
+	str := let.As[MyString](let.String(s))
+
+	s.Test("", func(t *testcase.T) {
+		t.Log(str.Get(t))
+	})
+}