Replace Fillable interface with a concrete wrapper

Signed-off-by: Solomon Hykes <sh.github.6811@hykes.org>

dagger/types.go

@@ -2,25 +2,31 @@ package dagger
 
 import (
 	"context"
+
+	cueflow "cuelang.org/go/tools/flow"
 )
 
 // Implemented by Component, Script, Op
 type Executable interface {
-	Execute(context.Context, FS, Fillable) (FS, error)
+	Execute(context.Context, FS, *Fillable) (FS, error)
 	Walk(context.Context, func(*Op) error) error
 }
 
 // Something which can be filled in-place with a cue value
-type Fillable interface {
-	Fill(interface{}) error
+type Fillable struct {
+	t *cueflow.Task
 }
 
-func Discard() Fillable {
-	return discard{}
+func NewFillable(t *cueflow.Task) *Fillable {
+	return &Fillable{
+		t: t,
+	}
 }
 
-type discard struct{}
-
-func (d discard) Fill(x interface{}) error {
-	return nil
+func (f *Fillable) Fill(x interface{}) error {
+	// Use a nil pointer receiver to discard all values
+	if f == nil {
+		return nil
+	}
+	return f.t.Fill(x)
 }