zsh completion to use word at cursor to run command

Question

I have been trying to grok how to write zsh completions, but he gap between understanding Create Basic ZSH-Command with Auto-Completion (super simple example and easy to understand) and zsh completions howto got to me and I couldn't figure this out. I am sure there are builtins to do this.

My goal: write a completion function that takes what I have typed, runs a command with it and spits out the result as the completion options. Most likely: take what I typed, grep through the files in some dir for that token and output the result.

This is as far as I have taken it:

runCommandOnFile() { 
  echo "Run on file: $1" 
}

_runCommandOnFile() { 
   # Does not show grep results like I expect.
   # compadd $(ls ~/dir/to/search | grep ${PREFIX})
   # Spits out list of all files in dir.
   compadd $(ls ~/dir/to/search)
} 
 
compdef _runCommandOnFile runCommandOnFile

I read that ${PREFIX} holds what was typed, but grepping with this isn't working as I expected.

I see that there are already existing functions to do regex etc, but I cannot see how that relates to an example like mine.

In bash for jump and mark, I achieved this result like so:

_marks_complete() {
   local curw=${COMP_WORDS[COMP_CWORD]}
   local word=${COMP_WORDS[COMP_CWORD]}
   COMPREPLY=()

   for file in $(find "${MARKPATH}" -type l -iname "*${word}*" -printf "%f\n") ; do
      # If the glob doesn't match, we'll get the glob itself, so make sure
      # we have an existing file

      COMPREPLY+=( "${file}" )
   done
   return 0
}

complete -F _marks_complete jump unmark