问题
If a language has control structures and variables, but no support for arrays, lists, memory access and allocation, etc, can it be Turing-complete?
Maybe if there was no limit to the amount of variables you can create, you can simulate arrays by creating variables like array_1, array_2, ... array_6000 and manually loop through them, and somehow create complex data structures and recursion?
Edit: Even if you cannot access variables by name manipulation (array_10+i is not allowed)?
回答1:
Certainly. Have a look at Lambda Calculus, which is one of the most minimal Turing Complete languages I've ever seen. Basically, all you have are lambdas (function literals); no assignment, no declaration, no data structures. It's all very very slimmed-down.
You can, however, simulate a linear data structure like a List by chaining functions together. It gets pretty verbose, but it's certainly possible and it's much nicer than having a large series of sequentially named variables.
Generally speaking, whether or not a language is Turing Complete has nothing to do with whether it has Arrays. Functional languages like SML and Haskell lack arrays, just like Lambda Calculus, and these are actually useful languages! Saying a language is "Turing Complete" is merely another way of saying that there is no Turing Computable function which cannot be expressed in said language. This is a surprisingly loose qualification, allowing many languages which would be completely impractical (like Lambda Calculus).
回答2:
There's plenty of Turing-complete languages that don't even have the notion of a "variable"! Memory access and allocations are implementation details, so they're completely irrelevant. You have to realize that Turing machines and Turing completeness are very theoretical concepts, useful for proving things, but completely divorced from the reality of actual hardware.
Paul Graham has written a long, but very, very interesting essay on the history of computer languages where he describes the two very different main traditions of computer languages:
- Lisp, Scheme, etc. - derived from theoretical considerations, very simple, yet conceptually powerful languages, but for the longest time impractical because of their complete disregard for what's easy and efficient to implement
- Assembler, FORTRAN, C and pretty much all "mainstream" languages - derived more or less directly from what the hardware could do, easy to implement, efficient, but for the longest time inferior to the (older!) Lisp family in terms of expressiveness.
It sounds like you know only the second tradition, but Turing completeness is a concept that originates from the same principles as the first tradition and makes little sense if you don't know those principles.
来源：https://stackoverflow.com/questions/1389981/can-a-language-be-turing-complete-without-any-support-for-arrays