give up on method notation to appease vint

exercises/practice/grains/.meta/example.vim

@@ -1,24 +1,24 @@
 " add two strings as numbers from right to left, as if by hand
 function! StringAdd(a, b) abort
   " zero-left-pad so the numbers are the same length
-  let len = [a:a->strlen(), a:b->strlen()]->max()
+  let len = max([strlen(a:a), strlen(a:b)])
   let a = printf('%0*s', len, a:a)
   let b = printf('%0*s', len, a:b)
   let result = ""
   let carry = 0
 
   for i in range(len - 1, 0, -1)
-    let c = carry + a->strpart(i, 1)->str2nr() + b->strpart(i, 1)->str2nr()
+    let c = carry + str2nr(strpart(a, i, 1)) + str2nr(strpart(b, i, 1))
     let result = $"{c % 10}{result}"
     let carry = c / 10
   endfor
-  return $"{carry}{result}"->trim('0', 1)
+  return trim($"{carry}{result}", '0', 1)
 endfunction
 
 " populate the squares, doubling each previous, cache in the script scope
 let s:grains = ['1']
 for i in range(1, 63)
-  eval s:grains->add(StringAdd(s:grains[i - 1], s:grains[i - 1]))
+  eval add(s:grains, StringAdd(s:grains[i - 1], s:grains[i - 1]))
 endfor
 
 " return the grains on square `number`