Syntax has dependencies. In (X . ARGS), the interpretation of ARGS
depends on X. So X should be validated* before ARGS is interpreted. Most
macros either obey this principle automatically or have relatively
benign violations**, but #%app is a bit special.
I agree with Carl.
Ryan
* Probably X doesn't need to be fully validated, just enough to justify
the decisions based on it.
** Here's a benign violation:
(rwhen body condition) => (when condition body)
If both body and condition contain syntax errors, the error in condition
is reported first, surprisingly. A slightly better way to write the
macro would be this:
(rwhen body condition) => (let ([c (lambda () condition)])
(when (c) body))
Expansion happens in the expected order, and the compiler can fix up the
code afterwards.
On 11/17/2010 02:19 PM, Matthias Felleisen wrote:
That's a fix for the symptom, and it occurred to me too.
Let's try to look at the large picture, too, instead of
just looking for bandaids for symptoms.
On Nov 17, 2010, at 4:17 PM, Carl Eastlund wrote:
Personally I would like to see the error here be that
define-struct/contract is not bound. If the order of either top level
expansion or #%app worked slightly differently, we could get that
error first and all would be clear.
Carl Eastlund
On Wed, Nov 17, 2010 at 4:15 PM, Matthias Felleisen
<matth...@ccs.neu.edu> wrote:
Isn't the true problem that we pretend uniformity of syntax and values? In this
case, we specifically pretend that functions and syntaxes may consume the same
kind of keyword-labeled arguments. Except that when you make a small mistake,
the brittleness of this arrangement shows up and you get WEIRD ERROR MESSAGES.
When systems work, nobody cares how they work. It's errors that make people
notice, and errors happen all the time.
On Nov 17, 2010, at 3:55 PM, Jon Rafkind wrote:
I found this error message confusing. The problem is I forgot to require
racket/contract.
#lang racket/base
(define-struct/contract foo ([a any/c]) #:transparent)
application: missing argument expression after keyword at: #:transparent
in: (#%app define-struct/contract foo ((a any/c)) #:transparent)
I was confused because I thought I had a (require racket/contract)
somewhere but apparently I didn't. I don't know if anything at all can
be done about it..
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