Ah, okay. Thanks Ryan.
So it looks like the exact same phase-2 x binding is being introduced
twice, first during local-expand, and then during processing of the
expanded code, which causes a conflict. (And that something is being
done to prevent that for phase 0 and 1 bindings.) Fortunately this is
the sort of problem that I'm unlikely to soon run into in practical
code, I should think. Was just doing some testing.
I conclude that long as I avoid top-level phase 2+ bindings, and I
guess beware of duplication of top-level phase 1+ side-effecting
operations, I should be okay with this approach.
On 12/09/13 22:16, Ryan Culpepper wrote:
Note that the problem is not restricted to submodules, and it has
nothing to do with the fact that x is defined in two phases. The same
error is raised by this program:
#lang racket/load
(module le-lang racket
(provide
(except-out (all-from-out racket) #%module-begin)
(rename-out [module-begin #%module-begin]))
(define-syntax (module-begin stx)
(syntax-case stx ()
((_ . bs)
(local-expand
#'(#%module-begin . bs)
'module-begin null)))))
(module le-module 'le-lang
(require (for-meta 2 racket/base))
(define x 0)
(begin-for-syntax
(begin-for-syntax
(define y 2))))
I conjecture that maybe the problem is that the module's environment (or
renames?) is not being cleared/reset at all phases when 'local-expand'
returns, only phases 0 and 1. But in a brief search of the expander code
I wasn't able to confirm my guess.
Ryan
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