faster external render check

pollen/render.rkt

@@ -233,16 +233,20 @@
 
 (define ram-cache (make-hash))
 
-(define (external-renderer)
-  (match  (setup:external-renderer)
-    [(list (? module-path? mod) (? symbol? render-proc-id))
-     (with-handlers ([exn:fail:filesystem:missing-module?
+(define (get-external-render-proc v)
+  (cond
+    [(and (list? v)
+          (eq? 2 (length v))
+          (module-path? (first v))
+          (symbol? (second v)))
+     (let ([mod (first v)]
+           [render-proc-id (second v)])
+       (with-handlers ([exn:fail:filesystem:missing-module?
                       (lambda (e) (error 'external-renderer "cannot open module ~a" mod))])
        (dynamic-require mod
                         render-proc-id
-                        (lambda () (error 'external-renderer "~a is not provided by ~a" render-proc-id mod))))]
-    [#f #f]
-    [(var v) (error 'setup:external-renderer "Value is not in the form '(module-path proc-id): ~a" v)]))
+                        (lambda () (error 'external-renderer "~a is not provided by ~a" render-proc-id mod)))))]
+    [else (error 'setup:external-renderer "Value is not in the form '(module-path proc-id): ~v" v)]))
 
 ;; note that output and template order is reversed from typical
 (define (render-to-file-base caller
@@ -270,7 +274,11 @@
       [(not render-cache-activated?) 'render-cache-deactivated]
       [else #false]))
   (when render-needed?
-    (define render-thunk (or maybe-render-thunk (λ () ((or (external-renderer) render) source-path template-path output-path)))) ; returns either string or bytes
+    (define render-thunk (or maybe-render-thunk
+                             (λ () ((or (let ([val (setup:external-renderer)])
+                                          (and val (get-external-render-proc val)))
+                                        render)
+                                    source-path template-path output-path)))) ; returns either string or bytes
     (define render-result
       (cond
         [render-cache-activated?