Yes, you want to be in an 'expression context. To get into one:
(define-syntax (local-expanding-transformer stx)
(cond
[(eq? 'expression (syntax-local-context))
(syntax-parse stx
[(_ e:expr)
(define-values [stx opaque]
(syntax-local-expand-expression #'e))
opaque])]
[else #`(#%expression #,stx)]))
Wrapping the result with `#%expression` is too late; you need to be in
an expression context already to successfully use
`syntax-local-expand-expression`.
At Fri, 27 Jan 2017 12:43:49 -0800, Alexis King wrote:
> 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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