add contracts to file/convertible

pkgs/racket-pkgs/racket-doc/file/scribblings/convertible.scrbl

@@ -64,15 +64,56 @@ should be considered standard:
         but for an PDF document}
 ]
 
-@defthing[prop:convertible struct-type-property?]{
+@defthing[prop:convertible 
+          (struct-type-property/c
+           (->i ([v convertible?] [request symbol?] [default default/c])
+                [result
+                 (case request
+                   [(text)
+                    (or/c string? default/c)]
+                   [(gif-bytes
+                     png-bytes
+                     png@2x-bytes
+                     ps-bytes
+                     eps-bytes
+                     pdf-bytes
+                     svg-bytes)
+                    (or/c bytes? default/c)]
+                   [(png-bytes+bounds
+                     png@2x-bytes+bounds
+                     eps-bytes+bounds
+                     pdf-bytes+bounds)
+                    (or/c (list/c bytes?
+                                  (and/c real? (not/c negative?))
+                                  (and/c real? (not/c negative?))
+                                  (and/c real? (not/c negative?))
+                                  (and/c real? (not/c negative?)))
+                          default/c)]
+                   [(png-bytes+bounds8
+                     png@2x-bytes+bounds8
+                     eps-bytes+bounds8
+                     pdf-bytes+bounds8)
+                    (or/c (list/c bytes?
+                                  (and/c real? (not/c negative?))
+                                  (and/c real? (not/c negative?))
+                                  (and/c real? (not/c negative?))
+                                  (and/c real? (not/c negative?))
+                                  (and/c real? (not/c negative?))
+                                  (and/c real? (not/c negative?))
+                                  (and/c real? (not/c negative?))
+                                  (and/c real? (not/c negative?)))
+                          default/c)]
+                   [else (or/c opaque-default/c any/c)])]))]{
 
-A property whose value should be a procedure of three arguments. The
-procedure is called when a structure with the property is passed to
-@racket[convert]; the first argument to the procedure is the
-structure, the second argument is a symbol for the requested
-conversion, and the third argument is a value to return (typically
-@racket[#f] if the conversion is not supported. The procedure's result
-depends on the requested conversion.}
+A property whose value is invoked by @racket[convert].
+
+The @racket[_v] argument to the procedure is the
+structure, the @racket[_request] argument is a symbol for the requested
+conversion, and the @racket[_default] argument is a value to return (typically
+@racket[#f] if the conversion is not supported). The procedure's result
+depends on the requested conversion, as above.
+
+The @racket[default/c] contract is one generated by @racket[new-α/c].}
 
 @defproc[(convertible? [v any/c]) boolean?]{
 
@@ -81,20 +122,30 @@ Returns @racket[#t] if @racket[v] supports the conversion protocol,
 
 @defproc[(convert [v convertible?] [request symbol?] [default any/c #f])
          (case request
-           [(text) (or/c string? (λ (x) (eq? x default)))]
-           [(gif-bytes png-bytes png@2x-bytes
-                       ps-bytes eps-bytes pdf-bytes svg-bytes)
-            (or/c bytes? (λ (x) (eq? x default)))]
-           [(png-bytes+bounds png@2x-bytes+bounds
-                              eps-bytes+bounds pdf-bytes+bounds)
+           [(text)
+            (or/c string? default/c)]
+           [(gif-bytes 
+             png-bytes
+             png@2x-bytes
+             ps-bytes
+             eps-bytes
+             pdf-bytes
+             svg-bytes)
+            (or/c bytes? default/c)]
+           [(png-bytes+bounds
+             png@2x-bytes+bounds
+             eps-bytes+bounds
+             pdf-bytes+bounds)
             (or/c (list/c bytes?
                           (and/c real? (not/c negative?))
                           (and/c real? (not/c negative?))
                           (and/c real? (not/c negative?))
                           (and/c real? (not/c negative?)))
-                  (λ (x) (eq? x default)))]
-           [(png-bytes+bounds8 png@2x-bytes+bounds8
-                               eps-bytes+bounds8 pdf-bytes+bounds8)
+                  default/c)]
+           [(png-bytes+bounds8
+             png@2x-bytes+bounds8
+             eps-bytes+bounds8
+             pdf-bytes+bounds8)
             (or/c (list/c bytes?
                           (and/c real? (not/c negative?))
                           (and/c real? (not/c negative?))
@@ -104,11 +155,14 @@ Returns @racket[#t] if @racket[v] supports the conversion protocol,
                           (and/c real? (not/c negative?))
                           (and/c real? (not/c negative?))
                           (and/c real? (not/c negative?)))
-                  (λ (x) (eq? x default)))]
-           [else any/c])]{
+                  default/c)]
+           [else (or/c opaque-default/c any/c)])]{
 
 Requests a data conversion from @racket[v], where @racket[request]
 indicates the type of requested data and @racket[default] is the value
 that the converter should return if it cannot produce data in the
-format indicated by @racket[request].}
+format indicated by @racket[request].
 
+The @racket[default/c] contract is one created by @racket[new-α/c]
+and it guarantees that the result of @racket[convert] is the given
+default argument (or @racket[#f] if one is not supplied).}

racket/collects/file/convertible.rkt

@@ -1,13 +1,57 @@
 #lang racket/base
 
-(provide prop:convertible convertible? convert)
+(require racket/contract)
+
+(define opaque-default/c (new-∀/c))
+
+(define default/c (or/c #f opaque-default/c))
+
+(define (make-result-contract request default)
+  (define default/c
+    (if (unsupplied-arg? default) default/c opaque-default/c))
+  (case request
+    [(text)
+     (or/c string? default/c)]
+    [(gif-bytes png-bytes png@2x-bytes ps-bytes eps-bytes pdf-bytes svg-bytes)
+     (or/c bytes? default/c)]
+    [(png-bytes+bounds png@2x-bytes+bounds eps-bytes+bounds pdf-bytes+bounds)
+     (or/c (list/c bytes?
+                   (and/c real? (not/c negative?))
+                   (and/c real? (not/c negative?))
+                   (and/c real? (not/c negative?))
+                   (and/c real? (not/c negative?)))
+           default/c)]
+    [(png-bytes+bounds8 png@2x-bytes+bounds8 eps-bytes+bounds8 pdf-bytes+bounds8)
+     (or/c (list/c bytes?
+                   (and/c real? (not/c negative?))
+                   (and/c real? (not/c negative?))
+                   (and/c real? (not/c negative?))
+                   (and/c real? (not/c negative?))
+                   (and/c real? (not/c negative?))
+                   (and/c real? (not/c negative?))
+                   (and/c real? (not/c negative?))
+                   (and/c real? (not/c negative?)))
+           default/c)]
+    [else (or/c opaque-default/c any/c)]))
+
+(provide
+ (contract-out
+  
+  [convertible? (-> any/c boolean?)]
+
+  [prop:convertible
+   (struct-type-property/c
+    (->i ([v convertible?] [request symbol?] [default default/c])
+         [result (request default) (make-result-contract request default)]))]
+
+  [convert
+   (->i ([v convertible?] [request symbol?])
+        ([default default/c])
+        [result (request default) (make-result-contract request default)])]))
+
 
 (define-values (prop:convertible convertible? convertible-ref)
   (make-struct-type-property 'convertible))
 
-(define (convert v target [default #f])
-  (unless (convertible? v)
-    (raise-type-error 'convert "convertible" 0 v target))
-  (unless (symbol? target)
-    (raise-type-error 'convert "symbol" 1 v target))
-  ((convertible-ref v) v target default))
+(define (convert v request [default #f])
+  ((convertible-ref v) v request default))