Vim 8.0 extended syntax highlighting for scheme

I use different versions of scheme (Gambit-C and Guile) and I have written a syntax file for (most of) Gambit-C's keywords. However I only want to use it when I am using files for Gambit-C.

Sourcing it from my vimrc does not work as the scheme syntax is sourced after my vimrc and If I source it using -s < scriptin > or -c < cmd >, it only works for the first file opened and I have to source it for each extra file/argument.

I saw this solution which solved my problem partially, but I do not really want vim to use the Gambit-C syntax for every scheme file. I also tried adding

if exists("b:is_gambit") || exists("is_gambit")
       "syntax extensions
         ....."
endif

and using it like was shown in the chicken scheme vim help but that did not work and then I realized that it did not seem to work for chicken scheme either (i.e. Setting "let b:is_chicken" in the ".vim/ftplugin/scheme.vim" file does not add chickens syntax to scheme)!!

Am I doing something wrong with the variables "(let b:is_chicken)" and (let b:is_gambit) or is there another way to make it work for selected scheme files eg making .vimrc source a file after all the files have loaded?

UPDATE: It turns out that for some reason, ".vim/ftplugin/scheme.vim" was not getting sourced and the variable "b:is_chicken was not set (this can be checked using ":scriptnames" or "echo b:is_chicken"). I then put "let b:is_chicken" in ".vim/syntax/scheme.vim" and the chicken scheme syntax worked. This does not distinguish between different scheme files though.

Solution

There seem to be three ways to do what I wanted, I needed to (try to) understand variables, autocmd and modelines

Adding to the to the vimrc that will be used for Gambit-C files (i.e. vi -u {vimrc} [Gambit-C files] ) any one of the following
```
         let g:is_gambit=1  "" This works on all the buffers 
```

            :autocmd BufRead *.scm let b:is_gambit "" This works for all the buffers but is redundant given the above.

and placing the syntax file of the form

   if exists("b:is_gambit") || exists("is_gambit")
         "syntax extensions
            ....."
   endif

as set up above to ".vim/after/syntax/scheme.vim".

Note: Setting b:is_gambit in the -u {vimrc} only sets it for the current buffer and g:is_gambit means all open (scheme .i.e. ".scm" ) buffers (if b:is_gambit is set in the ".vim/syntax/scheme.vim" it works in all buffers though). For variables, b = current buffer and g = global.

or......

Adding to the to the vimrc that will be used for Gambit-C files (i.e. vi -u {vimrc} [Gambit-C files] )
```
     :autocmd BufEnter *.scm source /path/to/file/gambitc.vim ""Note BufEnter not BufRead
```

Where gambitc.vim is the syntax file for Gambit-C.

Note: The gambitc.vim here only needs the extra syntax to be added to the scheme syntax

and......

As explained properly here setting modelines in your general vimrc and placing the Gambit-C syntax file in ".vim/syntax/" under a different name (e.g ".vim/syntax/gambitc.vim") and sourcing the original scheme syntax by placing this in the file
```
       :source /usr/share/vim/vim80/syntax/scheme.vim
```

or whatever the path to the scheme syntax file is.

Then in every file you want to use the Gambit-C syntax type

            ;; vim: filetype=gambitc

i.e comment this line from the current language and then write the statement.

If there is anything wrong with any of my solutions please let me know, I personally prefer the second one as it involves less work when porting it to different machines, though I suspect using the "g:is_gambit" method is more efficient