Why not be dependently typed?

Question

I have seen several sources echo the opinion that \"Haskell is gradually becoming a dependently-typed language\". The implication seems to be that with more and more languag

Answer 1

Dependent typing is really just the unification of the value and type levels, so you can parametrize values on types (already possible with type classes and parametric polymorphism in Haskell) and you can parametrize types on values (not, strictly speaking, possible yet in Haskell, although DataKinds gets very close).

Edit: Apparently, from this point forward, I was wrong (see @pigworker's comment). I'll preserve the rest of this as a record of the myths I've been fed. :P

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The issue with moving to full dependent typing, from what I've heard, is that it would break the phase restriction between the type and value levels that allows Haskell to be compiled to efficient machine code with erased types. With our current level of technology, a dependently typed language must go through an interpreter at some point (either immediately, or after being compiled to dependently-typed bytecode or similar).

This is not necessarily a fundamental restriction, but I'm not personally aware of any current research that looks promising in this regard but that has not already made it into GHC. If anyone else knows more, I would be happy to be corrected.