On Wed, Sep 25, 2013 at 7:59 AM, Sam Tobin-Hochstadt <sa...@cs.indiana.edu>wrote:
> On Wed, Sep 25, 2013 at 2:14 AM, Konrad Hinsen > <konrad.hin...@fastmail.net> wrote: > > > > > Maybe making the error message more specific like "Did you forget to > (require > > > (for-syntax syntax/parse)) ?" ? Or is it too specific? > > > > It would already help a lot if the error message said something about > > syntax-parse being undefined. But it complains about the pattern, > > which is kind of strange. It looks like syntax-parse is already there > > but incomplete. > > The reason it complains about the pattern is that `_` is already bound > -- `syntax-rules` and `syntax-case` both work at phase 0 without any > new requires. So Racket interprets everything as function application > until it gets to the `_`, which it tries to expand and gives an error > because it's not supposed to be used as an expression. Of course, you > didn't mean it as an expression, but as part of a `syntax-parse` > pattern, but because `syntax-parse` wasn't bound, it didn't get > recognized as a pattern. > > I think the error message could be better about making this clearer, > but I can't think of a good way that doesn't build-in `syntax/parse`. > > Sam > What I don't understand is why the implicit `#%app` added in front of `syntax-parse` doesn't blow up when it tries to expand its first argument -- the name `syntax-parse` -- and a use of `#%top` shows up. I thought `#%top` was supposed to blow up inside a module. If it blew up here, we'd see an error about the name `syntax-parse` before `#%app` ever got around to expanding the clauses with `_` in them. Somehow that error is being delayed, and the clauses are being treated as expressions as if the name `syntax-parse` had successfully expanded into an expression that might produce a function. If we can just make `#%top` cause a syntax error sooner here, we'll get intelligent error messages. --Carl
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