Using the Error Function

packages/strings/strings.grnd

@@ -46,14 +46,11 @@ endfun
 fun -string !mid -string &str -int &idx -int &len
     set &ans ""
     lesser $idx 0 &store
-    if $store %end
+    if $store %negend
     greater $len 0 &store
     if $store %loop
     return ""
     @loop
-        getstrsize $str &store
-        equal $store $idx &store
-        if $store %end
         getstrcharat $str $idx &store
         add $ans $store &ans
         add $idx 1 &idx
@@ -65,7 +62,8 @@ fun -string !mid -string &str -int &idx -int &len
     @return
     return $ans
 
-    @end
+    @negend
+    error "Cannot start mid at a negative index"
 endfun
 
 fun -string !inBrackets -string &str -char &left -char &right -int &instance