To what does typing correspond in a Turing Machine?

Question

I hope my question makes sense: Starting with the premise that the untyped $\lambda $ calculus is equivalent in power to a Turing machine, to what in a Turing machine does adding types to the $\lambda $ calculus correspond? Is there some kind of automaton analog to typing, whether static or dynamic?

Answer 1

I can't give you a direct answer for automatons, but for functions on numbers.

The untyped lambda calculus corresponds to $\mu$-recursive functions.

For typed systems, it naturally varies for different systems.

An interesting one is System F, also known as the polymorphic lambda calculus. There is a theorem that says that

A function (on natural numbers) is expressible in System F if and only if it can be proved in the second order Peano arithmetic that the function is total.

This means that in System F you can express basically all imaginable total functions.

There is a bit weaker system, Gödel's System T, for which there is a very similar theorem for first order Peano arithmetic. (However this system is not as nice as System F. In System F you can represent natural numbers, booleans etc. natively, while System T is constructed as the simply typed lambda calculus with externally added naturals and booleans. Also System F has type polymorphism, which makes it much more elegant in many cases.)

See also:

• Girard, Lafont and Taylor, Proofs and Types. Cambridge University Press, 1989, ISBN 0-521-37181-3.
• Is there an always-halting, limited model of computation accepting $R$ but not $RE$?

Answer 2

I'm going to say that typing doesn't correspond to anything for Turing Machines. Here's my reasoning: typing doesn't actually have to do with computation, and Turing Machines only model computations.

For the most part, the types of a program don't actually affect how that program runs. Rather, they're tools at compile time which are used for one of two reasons. One is to cause poorly typed programs not to compile. This is a purely compile-time operation, which doesn't affect how the resulting computation is carried out.

The second is to allow multiple functions to have the same name. This isn't really affecting the underlying computation, it's just letting us write programs in a way that's easier to understand.

Types aren't a tool of computation. They can be used for proofs, for understanding programs, for structuring them in ways that are easier to understand, but their presence doesn't change the fundamental way we perform the computation.