How to check if a string starts with another string in vimscript

Question

I know I can use regex matching with ~= and that stridx(haystack, needle) == 0 does work, but if I just want to verify that one string starts with another, I don't want to waste resources checking the rest of the haystack, which stridx will do.

What's the fastest way to compare the beginning of the string? I have my own answer below, but I suspect it can be done even better, perhaps by not creating a temporary string but by directly indexing the two strings?

Answer 1

fu! StartsWith(longer, shorter) abort
  return a:longer[0:len(a:shorter)-1] ==# a:shorter
endfunction