Day 08: No need for a hash when a function will do.

src/08.rkt

@@ -7,7 +7,7 @@
   (for/list ([entry (problem-input 8)])
     (match-let ([(list patterns outputs) (string-split entry " | ")])
       (list (map string->list (string-words patterns))
-            (map #{sort (string->list %) char<?} (string-words outputs))))))
+            (map string->list (string-words outputs))))))
 
 (define part1
   (~>> input
@@ -15,7 +15,7 @@
        (map #{count #{(or/c 2 3 4 7) (length %)} %})
        sum))
 
-(define (config patterns)
+(define ((config patterns) segs)
   (define (digit segments)
     (findf #{= (length %) segments} patterns))
   (define (digits segments)
@@ -30,22 +30,23 @@
   (define six   (set-union (set-subtract eight one) sixes))
   (define zero  (set-union (set-subtract eight three) (set-subtract eight four) one))
   (define two   (set-union (set-subtract zero  sixes) fives))
-  (hash (sort zero  char<?) #\0
+  (cond
-        (sort one   char<?) #\1
+    [(set=? zero  segs) #\0]
-        (sort two   char<?) #\2
+    [(set=? one   segs) #\1]
-        (sort three char<?) #\3
+    [(set=? two   segs) #\2]
-        (sort four  char<?) #\4
+    [(set=? three segs) #\3]
-        (sort five  char<?) #\5
+    [(set=? four  segs) #\4]
-        (sort six   char<?) #\6
+    [(set=? five  segs) #\5]
-        (sort seven char<?) #\7
+    [(set=? six   segs) #\6]
-        (sort eight char<?) #\8
+    [(set=? seven segs) #\7]
-        (sort nine  char<?) #\9))
+    [(set=? eight segs) #\8]
+    [(set=? nine  segs) #\9]))
 
 (define part2
   (for/sum ([entry input])
     (let ([mapping (config (first entry))])
       (~>> (second entry)
-           (map #{hash-ref mapping %})
+           (map mapping)
            list->string
            string->number))))