commit ecbbde87e233b4f99b04829a5d4ae76452b5b158
parent 1c10e60943c2e5c2dc3f010a63e02001201db83b
Author: Greg Hendershott <greghendershott@gmail.com>
Date: Fri, 2 Nov 2012 08:50:00 -0400
Split hash.refs into versions w and w/o error checking.
- Helps show the main logic w/o error-checking.
- Opportunity to make the case for syntax-parse.
Diffstat:
| M | main.rkt | | | 52 | +++++++++++++++++++++++++++++++++++++++------------- |
1 file changed, 39 insertions(+), 13 deletions(-)
diff --git a/main.rkt b/main.rkt
@@ -1100,12 +1100,43 @@ like JavaScript?
(require (for-syntax racket/syntax))
(define-syntax (hash.refs stx)
(syntax-case stx ()
+ ;; If the optional `default' is missing, assume it's #f.
+ [(_ chain)
+ #'(hash.refs chain #f)]
+ [(_ chain default)
+ (let ([xs (map (lambda (x)
+ (datum->syntax stx (string->symbol x)))
+ (regexp-split #rx"\\."
+ (symbol->string (syntax->datum #'chain))))])
+ (with-syntax ([h (car xs)]
+ [ks (cdr xs)])
+ #'(hash-refs h 'ks default)))]))
+;; Gives us "sugar" to say this:
+(hash.refs js.a.b.c)
+;; Try finding a key that doesn't exist:
+(hash.refs js.blah)
+;; Try finding a key that doesn't exist, specifying the default:
+(hash.refs js.blah 'did-not-exist)
+)
+
+It works!
+
+We've started to appreciate that our macros should give helpful
+messages when used in error. Let's try to do that here.
+
+@#reader scribble/comment-reader
+(i
+(require (for-syntax racket/syntax))
+(define-syntax (hash.refs stx)
+ (syntax-case stx ()
+ ;; Check for no args at all
[(_)
(raise-syntax-error #f "Expected (hash.key0[.key1 ...] [default])"
stx #'chain)]
[(_ chain)
#'(hash.refs chain #f)]
[(_ chain default)
+ ;; Check that chain is a symbol, not e.g. a number or string
(unless (symbol? (syntax-e #'chain))
(raise-syntax-error #f "Expected (hash.key0[.key1 ...] [default])"
stx #'chain))
@@ -1113,30 +1144,25 @@ like JavaScript?
(datum->syntax stx (string->symbol x)))
(regexp-split #rx"\\."
(symbol->string (syntax->datum #'chain))))])
+ ;; Check that we have at least hash.key
(unless (and (>= (length xs) 2)
(not (eq? (syntax-e (cadr xs)) '||)))
(raise-syntax-error #f "Expected hash.key" stx #'chain))
(with-syntax ([h (car xs)]
[ks (cdr xs)])
#'(hash-refs h 'ks default)))]))
-;; Gives us "sugar" to say this:
-(hash.refs js.a.b.c)
-)
-
-It works!
-
-We've started to appreciate that our macros should give helpful
-messages when used in error. We tried to do that here. Let's
-deliberately elicit various errors. Are the messages helpful?
-
-@i[
+;; See if we catch each of the misuses
(hash.refs)
(hash.refs 0)
(hash.refs js)
(hash.refs js.)
-]
+)
-Not too bad.
+Not too bad. Of course, the version with error-checking is quite a bit
+longer. Error-checking code generally tends to obscure the logic, and
+does here. Fortuantely we'll soon see how @racket[syntax-parse] can
+help mitigate that, in much the same way as contracts in normal
+Racket or types in Typed Racket.
Maybe we're not convinced that writing @racket[(hash.refs js.a.b.c)]
is really clearer than @racket[(hash-refs js '(a b c))]. Maybe we
