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Re: [tlaplus] How does TLC functions work under the hood?

On 8/13/21 6:12 AM, Victor Miraldo wrote:
I have some TLA+ spec that makes heavy use of functions and TLC takes a long time (minutes) to check that even the simplest property, i.e., `[](FALSE)`, doesn't hold. This made me question how TLC handles functions, but I can't find information elsewhere.

When I write [ x \in TypeX |-> f(x) ], is TLC generating all elements in `TypeX` then computing their image under `f` or does it wait until its called with an argument `m` to compute its image under `f`?

Is there a way to make TLA+ lazy, preventing it from ever generating `TypeX` in its entirety? Unfortunately we have some very large `TypeX`s in our generated code, such as the type of functions that receive lists and return lists.

Unfortunately, I can't use operators here because these need to be passed as arguments to mutually recursive operators.

Hi Victor,

I'd suggest to debug TLC [1] (look for tlc2.value.impl.FcnRcdValue), but the Toolbox's profiler [1] might be good enough to find out what is going on. Also, the TLA+ debugger [3] could help you introspect.

Btw. for "very large sets", TLC usually errors with:

 Evaluating assumption line ... module ... failed.
 Attempted to construct a set with too many elements (>1000000).

Since doesn't error out on you, you should ensure you're not chasing a red herring.


[1] https://github.com/tlaplus/tlaplus/tree/master/general/ide
[2] https://tla.msr-inria.inria.fr/tlatoolbox/doc/model/profiling.html
[3] https://youtu.be/6oMQYHogQek

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