# Expansion and "enriching"

**URL:** <https://racket.discourse.group/t/expansion-and-enriching/1294>\
**Category:** Questions & Answers\
**Tags:** syntax, expander\
**Created:** [September 8, 2022, 9:05pm UTC](https://racket.discourse.group/t/expansion-and-enriching/1294 "2022-09-08T21:05:34Z")\
**Posts on this page:** 2\
**Page:** 1

<div class="post-metadata">

**Author:** ![greghendershott](https://yyz2.discourse-cdn.com/free1/user_avatar/racket.discourse.group/greghendershott/32/98_2.png) [@greghendershott](https://racket.discourse.group/u/greghendershott)\
**Post date:** [September 8, 2022, 9:05pm UTC](https://racket.discourse.group/t/expansion-and-enriching/1294/1 "2022-09-08T21:05:34Z")

</div>

In Racket Mode I have a cache of fully expanded syntax. Some users of the cache need it to be "enriched", as done by `expand` (as opposed to `expand-syntax`).

In one case I want to "warm" the cache from a compiled load handler, which uses a compile handler, which expects un-enriched syntax (AFAICT) --- so I need to use `expand-syntax`. The question is, for the cache, can I reuse some of that work done by `expand-syntax`, i.e. avoid doing a whole other `expand` from scratch?

1. One idea is `(expand (expand-syntax stx))`. This seems safest (?). The idea is that `expand` will avoid expansion work per se (`stx` is already fully expanded by `expand-syntax`). But `expand` might need to navigate a huge fully-expanded syntax object (?). So probably I should thunk that, to do only if/as/when needed. This isn't terribly tricky, but it's a complication.

2. I wonder if `(namespace-syntax-introduce (expand-syntax stx))` would suffice (and be faster, so no need to delay thunk etc.) On Slack someone pointed out that `expand` is really `(expand-syntax (namespace-syntax-introduce stx))`, not `(namespace-syntax-introduce (expand-syntax stx))`. So this wouldn't be the same order. But does it matter? After all, this seems to be what `eval` (as opposed to `eval-syntax`) does, in case it gets un-enriched syntax from `expand-syntax`. Can't I just do the same?

And so I have a commit ready that does the first thing, and "it works" (more testing needed). But I just wonder if I'm needlessly complicating it and the second thing (or some third way?) would suffice.

---

<div class="post-metadata">

**Author:** ![greghendershott](https://yyz2.discourse-cdn.com/free1/user_avatar/racket.discourse.group/greghendershott/32/98_2.png) [@greghendershott](https://racket.discourse.group/u/greghendershott)\
**Post date:** [September 12, 2022, 6:17pm UTC](https://racket.discourse.group/t/expansion-and-enriching/1294/2 "2022-09-12T18:17:34Z")

</div>

> In one case I want to "warm" the cache from a compiled load handler, which uses a compile handler, which expects un-enriched syntax (AFAICT)

After thinking more and looking at more Racket source, I think it's fine for the compile handler to just go ahead and do the full `expand`. The default compile handler does that anyway, `(expand-syntax (namespace-syntax-introduce stx))` ~= `(expand stx)`.

So it's fine both to put that same value in the cache and return it from the compile handler.