Given that Turnstile makes extremely heavy use of local-expand,
it’s easy to imagine that the macro expander could traverse a single
piece of syntax dozens of times when it really only needs to traverse
it once. Looking at the docs, it seems like syntax-local-expand-expression
is precisely designed for this sort of thing, but I have no idea
how to properly use it. I tried writing the simplest possible macro
I could think of, which expands a single expression and produces
it:
(define-syntax-parser local-expanding-transformer
[(_ e:expr)
(define-values [stx opaque]
(syntax-local-expand-expression #'e))
opaque])
However, this doesn’t seem to work:
> (local-expanding-transformer (+ 1 2))
#%app: expanded syntax not in its original lexical context
(extra bindings or scopes in the current context)
in: (#%app + (quote 1) (quote 2))
This seems to happen because the resulting expression is expanded
in an internal definition context rather than an expression context,
because this works, instead:
> (#%expression (local-expanding-transformer (+ 1 2)))
3
However, trying to move that use of #%expression into the expansion
of local-expanding-transformer itself does not alleviate the problem:
(define-syntax-parser local-expanding-transformer
[(_ e:expr)
(define-values [stx opaque]
(syntax-local-expand-expression #'e))
#`(#%expression #,opaque)])
> (local-expanding-transformer (+ 1 2))
#%app: expanded syntax not in its original lexical context
(extra bindings or scopes in the current context)
in: (#%app + (quote 1) (quote 2))
Considering this, does this mean that the only situation in which
it’s safe to use syntax-local-expand-expression is when
(syntax-local-context) is 'expression? Is there any way for me to
force the result of the macro to be expanded in an expression
context? Why doesn’t wrapping the result with #%expression work?
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