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[tlaplus] Re: Supporting Action Composition in TLC

Thank you, that's helpful background information. I would be curious to know what you mean when you say that ENABLED and action composition are "mathematically ugly" operators. From a cursory glance, I might guess that you consider them "ugly" because they are in some way an "extension" to a more primitive core of TLA. For example, in the Action Operators section (16.2.3) of Specifying Systems, you give definitions for ENABLED and the composition operator, both of which cannot be defined in terms of existing constructs. Their definitions also appear to rely on quantification over states, which seems distinct from other action operator definitions.  

It seems that most common, practically useful TLA formulas can be written in terms of the prime construct (') and the box operator (☐). ENABLED and action composition introduce exceptions to this. Temporal operators like \EE would seem to be another exception, but perhaps you have similar opinions on their aesthetics based on your "stone in the soup" comments from [1]. 

I also recall that the definitions of weak and strong fairness are given in terms of ENABLED, so perhaps that was one motivator for inclusion of the "necessary evil", but I am mostly conjecturing here.

[1] https://lamport.azurewebsites.net/tla/hiding-and-refinement.pdf

On Wednesday, May 6, 2020 at 12:43:49 PM UTC-4, William Schultz wrote:
As I understand it, TLC does not currently support the TLA+ action composition operator (mentioned in section 7.3 of Specifying Systems). It appears that it does have the ability to parse the operator within a spec, though. I am curious if there are any fundamental difficulties in implementing support for it. Perhaps it was never implemented simply because it was low priority or didn't seem useful enough for practical specs. I was recently working on a series of specifications where I may have found support for the composition operator useful. Essentially, I wanted to take one spec that modeled things at a fine grain of atomicity and compare it to a version of the spec where several lower level actions were executed as a single, coarse grained atomic step. I was able to manually implement the coarse grained version of the spec, but I felt it may have been easier and clearer if I was able to describe the coarse steps as the composition of fine grained steps. Any background information or thoughts on this would be appreciated. I can also open a Github issue if this might be a feasible feature to implement in the future.

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