Extend special characters using a function rather than a dict.

Based on feedback from David Van Horn.

scribble-doc/scribblings/scribble/renderer.scrbl

@@ -2,7 +2,6 @@
 @(require scribble/manual 
           "utils.rkt" 
           (for-label racket/class
-                     racket/dict
                      scribble/render
                      scribble/xref))
 
@@ -397,11 +396,14 @@ are own their own pages. A value of @racket[0] is treated the same as
 
 Specializes a @racket[render<%>] class for generating Latex input.}}
 
-@defparam[extra-character-conversions convs (dictof char? string?)]{
-Maps (special) characters to strings corresponding to the Latex code that
-should be used to render them. Scribble already converts many special
-characters to the proper Latex commands. This parameter should be used in case
-you need characters it does not support yet.
+@defparam[extra-character-conversions convs (-> char? (or/c string? #f))]{
+Function that maps (special) characters to strings corresponding to the Latex
+code that should be used to render them. This function should return false for
+any character it does not know how to handle.
+
+Scribble already converts many special characters to the proper Latex
+commands. This parameter should be used in case you need characters it does not
+support yet.
 }
 
 @; ----------------------------------------

scribble-lib/scribble/latex-render.rkt

@@ -3,7 +3,6 @@
          "latex-properties.rkt"
          "private/render-utils.rkt"
          racket/class
-         racket/dict
          racket/runtime-path
          racket/port
          racket/string
@@ -49,7 +48,7 @@
 (define-runtime-path skull-tex "scribble-skull.tex")
 (define skull-style (make-style #f (list (tex-addition skull-tex))))
 
-(define extra-character-conversions (make-parameter (make-hash)))
+(define extra-character-conversions (make-parameter (λ (c) #f)))
 
 (define (render-mixin % #:image-mode [image-mode #f])
   (class %
@@ -1001,7 +1000,7 @@
                      [else
                       (if ((char->integer c) . > . 127)
                           ;; first, try user-defined conversions
-                          (or (dict-ref convs c #f)
+                          (or (convs c)
                               ;; latex-prefix.rkt enables utf8 input, but this does not work for
                               ;; all the characters below (e.g. ∞). Some parts of the table
                               ;; below are therefore necessary, but some parts probably are not.