Add command :NixShell

Add a command to call 'nix-shell' and then update the editor's
environment as if it was inside the new shell.

It's sometimes useful to fire up a nix-shell just to download a package
for using it once. It's annoying however when it's a library or any
other dependency of ':make', which can't be called from inside the
shell.

Example:

    :NixShell -p hello
    :term hello

There's no rollback command but it could be written if it's ever needed.
Calling the commande several times will nest environments the same way
as opening a shell inside a shell.

Author: Julow

plugin/nix.vim

@@ -1 +1,45 @@
+function! nix#find_drv_position()
+  let line = search("description")
+  if line == 0
+    let line = search("name")
+  endif
+  if line == 0
+    echo "error: could not find derivation"
+    return
+  endif
+
+  return expand("%") . ":" . line
+endfunction
+
+function! nix#edit(attr)
+  let output = system("nix-instantiate --eval ./. -A " . a:attr . ".meta.position")
+  if match(output, "^error:") == -1
+    let position = split(split(output, '"')[0], ":")
+    execute "edit " . position[0]
+    execute position[1]
+    " Update default command to nix-build.
+    let b:dispatch = 'nix-build --no-out-link -A ' . a:attr
+  endif
+endfunction
+
+" Takes a list of lines of the form 'key=val'.
+function! nix#set_env(lines)
+  for l in a:lines
+    let [ key; val ] = split(l, "=", 1)
+    call setenv(key, join(val, "="))
+  endfor
+endfunction
+
+" See command :NixShell
+function! nix#add_pkg_to_env(args)
+  echo "nix-shell " . a:args
+  let l:env = systemlist("nix-shell --run env " . a:args)
+  call nix#set_env(l:env)
+  echo "Environment updated"
+endfunction
+
 command! -bang -nargs=* NixEdit call nix#edit(<q-args>)
+
+" Call 'nix-shell' with the given arguments and update the environment as if
+" we were inside the spawned shell.
+command! -nargs=* NixShell call nix#add_pkg_to_env(<q-args>)