Tic tac types: a gentle introduction to dependently typed programming (functional pearl).

Tic-Tac-Toe is a simple, familiar, classic game enjoyed by many. This pearl is designed to give a flavour of the world of dependent types to the uninitiated functional programmer. We cover a journey from Tic-Tac-Terrible implementations in the harsh world of virtually untyped |Strings|, through the safe haven of vectors that know their own length, and into a Tic-Tac-Titanium version that is too strongly typed for its own good. Along the way we discover something we knew all along; types are great, but in moderation. This lesson is quickly put to use in a more complex recursive version.