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I thought about how in lambda calculus (and many implementations of functional programming languages) function (lambda) application and lambda itself, as a construct, are "primitive things", usually somehow implemented by an interpeter. Then I thought, can you "boil-down" these two things to more primitve stuff. For instance, we have a following expression (apply is usually implicit by the syntax convetions, but whatever)

(apply (\x.\y.x) (a b))

The interpreter:
1. Constructs a new environment, where arguments are bound to lambda's terms, i.e. new_env = this_env.append({"x":a, "y":b, "body":"x"})
2. Performs a rewrite of the whole application term with lambda's body, i.e. new_env["body"]

Given only the environment manipualtion "primitives", like: "construct", "append", and "get", doesn't that make whole lambda calculus just a clever trick to hide memory ("tape") mutations? Now I know that turing machine and lambda calculus are equivalent, but is there something more to LC than just what I've described? What have I missed?

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  • $\begingroup$ The interpreter needs to treat free variables too: en.wikipedia.org/wiki/…. Nowadays almost everybody uses lexical scopes, so you'd need to create a closure first. $\endgroup$
    – Anton Trunov
    Apr 30 '16 at 16:44
  • $\begingroup$ Again, closure is an object (environment) with bindings. This just an additional "complication" to this memory-management abstraction, isn't it? $\endgroup$
    – artemonster
    Apr 30 '16 at 16:50
  • $\begingroup$ Well, anything that is workable on a computer for which any notion of state is defined, is actually a memory manipulation algorithm. Can't see how can it be the other way? $\endgroup$
    – noncom
    Sep 21 '18 at 11:38
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That seems reasonable to me, although I've never thought of other that way. The critical point is that LC essentially involves variables and substitution. So I wouldn't say it "hides" memory ops, exactly, that's just how LC substitution translates to Turing machinery. On the other hand, substitution is not primitive - combinatory logic eliminates both variables and substitution. So you might say LC introduces rather than hides something.

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  • $\begingroup$ But again, combinators operate on stack of stacks. They just hide all that symbol->value mapping and make it implicit. They do not eliminate the "tape", right? $\endgroup$
    – artemonster
    Apr 30 '16 at 20:07
  • $\begingroup$ Well, I confess I've never tried to think about how combinators would translate into Turing machines. But a Turing Machine without a tape would not be a Turing Machine, I think. More to the point: if you don't have vars and substitution then you don't have symbol->value bindings. At least not formally, you can still philosophically interpret your syms, but such extra-syntactic semantic bindings are not relevant to computation. So you still have a tape, but nothing is hidden or implicit. Dealing with the tape is just what computation is, in part. At least that's my understanding. $\endgroup$
    – user48832
    Apr 30 '16 at 20:15

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