There is a good chance that forA is not the function you want to use here. That’s because the function you pass to it is only allowed to depend on the argument (quantity), and the only form of “sequence”, or things happening in a particular order, comes from the actions you are performing with the do. Specifically, variable binding is not such an action; it is purely a local alias for a value, and cannot escape its scope of definition. (To put it another way, DAML functions must be true functions, whose only “effect” is in accepting arguments and returning values.)
You might be comfortable with forA from examples. However, it is not a “one size fits all” function for working across the aggregate of a list; it is used in those examples because it hides some details of lists that are sometimes useful, but not always. If there is any “one size fits all” function for lists, it’s foldr or foldl as @cocreature used previously, but you won’t see foldr in most examples, because it is much nicer in DAML to use a function that more closely fits your use case, and foldr is far too powerful and general-purpose for that.
To determine what the proper function to use instead of forA is, first decide: are you performing any actions from the function you’re passing in, such as contract exercises or creates? If you are, a good starting point is the DA.Action module; if you are not, check the DA.Foldable module instead. Regardless, the starting point is to make that determination.