This issue used to be called "zsh tab completion". I have decided that the best solution to this problem is to develop a generic tab completion function.
Tab completion has been parameterized in branch issue5 (https://github.com/resultsreturned/to/tree/issue5)
_to: generic tab completion function
_to_bash: bash tab completion function
_to_zsh: zsh tab completion function (non functional)
Requires an implementation of compgen (specified with $TO_COMPGEN). If this is problematic, we need a version of compgen that is shell agnostic.
I have written a potential fix in the issue5 branch, but I am unable to test the zsh fix.
Renewed (zsh only)
fixed and tested tab completion for zsh
The master branch and issue5 branch have been merged. Development on this issue will continue, but the functionality of zsh tab completion in the issue5 branch was good enough to be migrated back to master.
The _to completion generation function is now generic and does not depend of bash specific tools as of 05b10be. Any new shells that need support should open new issues.