Why does (indent-region ...) insert a 't' into my buffer?

Question

I'm using this yasnippet:

# -*- mode: snippet -*-
# name : case : {...}
# key: case
# --
`(indent-region (- (point) 20) (+ (point) 20) nil)`case ${2:constexpr}: ${3:\{}
    $0
    break;
${3:$(if (string-match "\{" yas-text) "\}" "")}

I think it's some default, but I'm not sure. Until recently, this snippet worked fine, but now I get a t inserted into my buffer, and I think it comes from (indent-region):

        break;
t      case constexpr: {

          break;
        }
      case

How do I get rid of this t?

Answer 1

The meaning of backquotes in a snippet is to insert the value of the form within the backquotes into the snippet expansion. However, indent-region works by side-effect, and in recent Emacs versions happens to return t. As politza suggested in the comments, you can use (progn (indent-region ...) nil) to drop the return value.

However, this still performs a side effect inside backquotes which will stop being supported in some future version (it triggers a warning in versions 0.11 and higher). In this case, the indentation effect can be written much clearer by binding yas-also-auto-indent-first-line:

# -*- mode: snippet -*-
# name : case : {...}
# key: case
# expand-env: ((yas-also-auto-indent-first-line t))
# --
case ${2:constexpr}:${3: \{}
    $0
    break;
${3:$(if (string-match "\{" yas-text) "\}" "")}

This modification has just been accepted at https://github.com/AndreaCrotti/yasnippet-snippets/pull/176, and will come with the next yasnippet version (0.12), at least for MELPA packages which integrate snippets from that repo. For other examples of avoiding side effects within backquotes, see yasnippet truncates clipboard contents.