On 1/8/07, Tomasz Zielonka
On Mon, Jan 08, 2007 at 08:02:36AM -0500, Lennart Augustsson wrote:
So it sounds to me like the (terminating) type checker solves the halting problem. Can you please explain which part of this I have misunderstood?
Perhaps you, the user, have to encode the proof of halting in the way you construct the term?
The Terminating datatype takes three parameters: 1. A term in the untyped lambda calculus 2. A sequence of beta reductions 3. A proof that the result of the beta reductions is normalized. Number 2 is the hard part. For a term that calculated the factorial of 5, the list in part 2 would be at least 120 items long, and each one is kind of a pain. GHC's type checker ends up doing exactly what it was doing before: checking proofs. Jim