refactor: String::concat implementation

Use pattern matching for more concise string concatenation logic

Author: bobzhang

string/string.mbt

@@ -64,29 +64,28 @@ pub fn String::concat(
   strings : Array[String],
   separator~ : String = ""
 ) -> String {
-  if strings.length() == 0 {
-    ""
-  } else {
-    let hd = strings[0]
-    let tl = strings[1:]
-    let mut size_hint = hd.length()
-    for s in tl {
-      size_hint += s.length() + separator.length()
-    }
-    size_hint = size_hint << 1
-    let buf = StringBuilder::new(size_hint~)
-    buf.write_string(hd)
-    if separator == "" {
+  match strings {
+    [] => ""
+    [hd, .. tl] => {
+      let mut size_hint = hd.length()
       for s in tl {
-        buf.write_string(s)
+        size_hint += s.length() + separator.length()
       }
-    } else {
-      for s in tl {
-        buf.write_string(separator)
-        buf.write_string(s)
+      size_hint = size_hint << 1
+      let buf = StringBuilder::new(size_hint~)
+      buf.write_string(hd)
+      if separator == "" {
+        for s in tl {
+          buf.write_string(s)
+        }
+      } else {
+        for s in tl {
+          buf.write_string(separator)
+          buf.write_string(s)
+        }
       }
+      buf.to_string()
     }
-    buf.to_string()
   }
 }