Make vim respect the tab width settings

indent/dune.vim

@@ -2,14 +2,36 @@
 " Language: dune
 " Maintainers:  Markus Mottl         <[email protected]>
 " URL:          https://github.com/ocaml/vim-ocaml
-" Last Change:  2020 Dec 31
+" Last Change:  2024 Nov 8 -  Make vim respect the tab width settings (Stephen Sherratt)
+"               2020 Dec 31
 
 if exists("b:did_indent")
  finish
 endif
 let b:did_indent = 1
 
+" A rough approximation of the indentation behaviour implemented by
+" `dune format-dune-file`.
+function! DuneIndent()
+    let prev_line = getline(v:lnum - 1)
+    let prev_indent = indent(v:lnum - 1)
+    let current_indent = prev_indent
+    for i in range(len(prev_line))
+        if prev_line[i] == '('
+            let current_indent += &shiftwidth
+        endif
+        if prev_line[i] == ')'
+            let current_indent -= &shiftwidth
+        endif
+    endfor
+    return current_indent
+endfunction
+
 " dune format-dune-file uses 1 space to indent
-setlocal softtabstop=1 shiftwidth=1 expandtab
+" Explicitly set "lisp" since without this setting vim will not auto indent
+" sexp files like dune files at all. When "lisp" is enabled, vim doesn't
+" respect softtabstop or shiftwidth by default, so a custom indentexpr is
+" needed.
+setlocal softtabstop=1 shiftwidth=1 expandtab lisp indentexpr=DuneIndent() lispoptions=expr:1
 
 let b:undo_indent = "setl et< sts< sw<"