more simplification

day01-easy/main.scm

@@ -22,14 +22,14 @@
     current
     (reduce (cdr list) reducer (reducer current (car list)))))
 
-(define (combine list combiner)
+(define (combine a b combiner)
   (if
-    (null? (car list))
-    (cdr list)
+    (null? a)
+    b
     (if
-      (null? (cdr list))
-      (car list)
-      (combiner (car list) (cdr list)))))
+      (null? b)
+      a
+      (combiner a b))))
 
 (define (sum numbers) (reduce numbers + 0))
 
@@ -43,22 +43,20 @@
 (define (is-numeric-char char) (if (char-numeric? char) #t #f))
 
 (define (first-last list predicate)
-  (if
-    (null? list)
-    '()
+  (reduce
+    (map list (lambda (entry)
+      (if (predicate entry) (repeat entry 2) '())))
+    (lambda (current accumulator)
       (combine
-      (cons
-        (if
-          (predicate (car list))
-          (repeat (car list) 2)
-          '())
-        (first-last (cdr list) predicate))
+        current
+        accumulator
         (lambda (current rest)
-        (cons (car current) (cdr rest))))))
+          (cons (car current) (cdr rest)))))
+    '()))
 
 (define (solve-line line)
-  (
-    (lambda (first-last-result) (string->number (list->string first-last-result)))
+  ((lambda (first-last-result)
+    (string->number (list->string first-last-result)))
     (first-last (string->list line) is-numeric-char)))
 
 (define (solve-all lines)