I don't know. But - and this is my interpretation, not some summary of another talk - it sounds like Simon Peyton Jones, who is one of the people who are responsible for the poor state of the code, already answered that: "The difficult stuff is envisioning and being very clear about what you’re trying to do." Presumably, wherever his skills lie, they don't lie in envisioning a better state and refactoring the code towards that goal (to him, it was "difficult stuff", and left aside as he implemented changes that had a greater impact on its users). Afaik, for a long time GHC was maintained by the same two or three people so it probably shows the imprint of individual developers much more than your average compiler of similar size. Now development is much more open and this kind of technical debt is getting paid off.