$VIMRUNTIME/indent

indent/ contains Vim scripts that assist the editor of a given file type with indenting the file.

Rules for an indent file

" Only load this indent file when no other was loaded yet.

if exists("b:did_indent")
   finish
endif

let b:did_indent = 1

To trigger the indenting after typing a key word like endif, add the word to the cinkeys option with +=.

You normally set indentexpr to evaluate a function and then define that function.

That function only needs to be defined once for as long as Vim is running. Add a test if the function exists and use ":finish", like this:

if exists("*GetMyIndent")
   finish
endif

To test the indent file, see testdir/README.txt.