ftplugin/typescript.vim: Make `gf` open `index.ts` if path is directory

In TypeScript and Node.js, you can import a file `./directory/index.ts`
by importing `./directory`.

I wanted `gf` and friends to open the `index` file if the import refers
to a directory. By default, `gf` opens the directory using Netrw.

At first tried to do this by setting 'includeexpr' as follows:

    setlocal includeexpr=<SID>FindFile(v:fname)

    function! s:FindFile(fname)
      echom 'FindFile: ' . a:fname
      if filereadable(a:fname)
        return a:fname
      endif

      return a:fname . '/index'
    endfunction

This gave me the following error:

    E120: Using <SID> not in a script context: <SID>FindFile

so I tried removing `<SID>`:

    setlocal includeexpr=FindFile(v:fname)

    function! FindFile(fname)
      echom 'FindFile: ' . a:fname
      if filereadable(a:fname)
        return a:fname
      endif

      return a:fname . '/index.ts'
    endfunction

The problem was, my 'includeexpr' function wasn't getting executed every
time I used `gf`, only some times. And it never ran when I used `gf` on
a directory.

After asking on Freenode#vim, 'crose' made the following suggestion:

    augroup typescript_maybe_append_filename
        au!
        au BufEnter * if expand('<amatch>')->isdirectory()
            \ | call s:maybe_append_filename()
            \ | endif
    augroup END

    fu s:maybe_append_filename() abort
        if getbufvar('#', '&ft', '') == 'typescript'
            exe 'e ' .. expand('%:p') .. 'index.ts'
        endif
    endfu

This works, but it adds the Netrw directory buffer to the jumplist, and
feels more like a mitigation than a solution.

I looked at the following language ftplugins for inspiration:

* .../vim/8.2.1650/share/vim/vim82/ftplugin/ruby.vim
* https://github.com/moll/vim-node/blob/master/autoload/node.vim

These prompted me to override `gf` with a custom mapping since it looked
like 'includeexpr' wasn't going to be the right tool for this. The
`s:FindFile()` function has similar logic, but is now written as a map
rhs function, taking into account the different ways of invoking `gf`.

When defined as a script-local function, I at first kept getting this
error using `gf` on a directory:

    E127: Cannot redefine function <SNR>111_FindFile: It is in use

Thanks to Ingo Karkat
(https://stackoverflow.com/users/813602/ingo-karkat) and this Stack
Overflow answer which describes how to keep the function in the same
file (so I didn't have to move it to an autoload function) and still
prevent it from being redefined:
https://stackoverflow.com/questions/22633115/why-do-i-get-e127-from-this-vimscript/22633702#22633702

I needed to prepend `<C-v>` to the `<C-w>` characters in the map
function calls because these would actually run in the command line,
deleting the preceding word, and messing up the mapping. In order to
pass those `<C-w>` characters in the string arguments, I needed to
escape them with `<C-v>`, even though it resulted in non-obvious looking
code.

1 files changed, 38 insertions, 0 deletions

diff --git a/ftplugin/typescript.vim b/ftplugin/typescript.vim
index c68494a..94b9c9e 100644
--- a/ftplugin/typescript.vim
+++ b/ftplugin/typescript.vim
@@ -1,3 +1,41 @@
 " TypeScript vim settings
 
 setlocal commentstring=//\ %s
+
+setlocal suffixesadd+=.ts
+
+let b:undo_ftplugin = ''
+
+
+nnoremap <silent> <buffer> gf
+	\ :<C-u>call <SID>FindFile(v:count1, expand('<cfile>'), 'find', 'gf')<CR>
+nnoremap <buffer> <C-w>f
+	\ :<C-u>call <SID>FindFile(v:count1, expand('<cfile>'), 'sfind', '<C-v><C-w>f')<CR>
+nnoremap <silent> <buffer> <C-w><C-f>
+	\ :<C-u>call <SID>FindFile(v:count1, expand('<cfile>'), 'sfind', '<C-v><C-w><C-v><C-f>')<CR>
+nnoremap <silent> <buffer> <C-w>gf
+	\ :<C-u>call <SID>FindFile(v:count1, expand('<cfile>'), 'tabfind', '<C-v><C-w>gf')<CR>
+
+let b:undo_ftplugin .= 'nunmap <buffer> gf'
+let b:undo_ftplugin .= '| nunmap <buffer> <C-w>f'
+let b:undo_ftplugin .= '| nunmap <buffer> <C-w><C-f>'
+let b:undo_ftplugin .= '| nunmap <buffer> <C-w>gf'
+
+
+if exists('*s:FindFile')
+	finish
+endif
+
+" If `fname` is a directory, open fname/index.ts using `command`. Otherwise,
+" run `map` with `count`.
+function! s:FindFile(count, fname, command, map)
+	let relative_file = expand('%:h') . '/' . a:fname
+
+	if isdirectory(relative_file)
+		execute a:command . ' ' . relative_file . '/index.ts'
+
+		return
+	endif
+
+	execute 'normal! ' . a:count . a:map
+endfunction