# 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 TypeXs 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.

M.

[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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