add list of digits into integer recipe

Author: jcubic

recipes/convert-list-of-digits-to-integer.md

@@ -0,0 +1,37 @@
+# Convert list of digits into integer
+
+## Problem
+
+I have a list of integers that are digits of a number, the digits may start
+with symbol `-`. So this is the reverse operation of the function `integer->list`
+from recipe [Convert integer to list of digits](/convert-integer-to-list-of-digits/).
+
+## Solution
+
+### Using string->number and number->string
+
+```scheme
+(define (list->integer lst)
+  (string->number (apply string-append
+                         (map (lambda (item)
+                                (if (and (symbol? item) (symbol=? item '-))
+                                    "-"
+                                    (number->string item)))
+                              lst))))
+```
+
+Credit: [Jakub T. Jankiewicz](https://jcubic.pl/me)
+
+## Usage
+
+Testing if the both functions return original value
+
+```scheme
+(let ((numbers '(1000 -1234)))
+  (for-each (lambda (number)
+              (display (eq? number (list->integer (integer->list number))))
+              (newline))
+            numbers))
+;; ==> #t
+;; ==> #t
+```