feat(directive): add func resolver

Signed-off-by: Christian Stewart <[email protected]>

directive/func-resolver.go

@@ -0,0 +1,24 @@
+package directive
+
+import "context"
+
+// FuncResolver resolves a directive with a function.
+type FuncResolver struct {
+	fn func(ctx context.Context, handler ResolverHandler) error
+}
+
+// NewFuncResolver constructs a new FuncResolver.
+func NewFuncResolver(fn func(ctx context.Context, handler ResolverHandler) error) *FuncResolver {
+	return &FuncResolver{fn: fn}
+}
+
+// Resolve resolves the values, emitting them to the handler.
+func (r *FuncResolver) Resolve(ctx context.Context, handler ResolverHandler) error {
+	if r.fn == nil {
+		return nil
+	}
+	return r.fn(ctx, handler)
+}
+
+// _ is a type assertion
+var _ Resolver = ((*FuncResolver)(nil))