Re: Hindley-Milner, this one is really really tough and I am not convinced either way yet. The problem is that it's so easy to whip something up in Ruby and then write a slick rspec suite that ostensibly shows that it does what its supposed to. At the time you launch this there is no value in strict type safety because it's easily testable and self-evident that it works.

The value of type safety comes in tiny increments over time. Week after week, month after month, year after year. If your project survives 10 years I am quite confident that the effort of implementing in a strictly typed language would have paid for itself by the number of bugs prevented and the number of wiring tests that didn't have to be written/maintained. Of course if it's just a solo project and you keep the code under control it's possible this is not true, but with developer and code churn I'd say this is practically impossible. The more important question is will this code base even need to live for 10 years in the first place?