What's the easiest way to add custom filename autocomplete behavior for a command?

Question

I want to add some rudimentary filename completion for programs like ghc, ghci (Haskell compilers), lyx (typesetting program), namely that hitting [tab] after one of the above should offer autocomplete suggestions for files ending in ".hs" or ".lyx".

What's the easiest way to do this in zsh?

Answer 1

compdef '_files -g "*.hs *.lyx"' ghc ghci lyx

See man zshcompsys | less -p '^\s*compdef \[' and generally man zshcompsys.