How to express this set comprehension in TLA+?

[hengx...@gmail.com]

I want to express the following set comprehension (as a simplied example)

{<<x, y>> : x \in 1 .. 2, y \in 1 .. 2, y # x}

in TLA+ and expect the result {<<1, 2>>, <<2, 1>>}.

Note that the third condition "y # x" uses variable "x".

However, I got a parse error.

How should I correctly express it in TLA+?

Best regards,

hengxin

[Martin]

Hello Hengxin,

The { e: x \in S} pattern is treated differently, you need to move the domain
constraints to the front:

P == { <<x,y>> \in 1..2 \X 1..2 : x # y }

Please refer also to the Specifying Systems book p.299 where you can see the
different shapes. I hope that helps :)

kind regards,
Martin

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[hengx...@gmail.com]

Hi Martin,

Thanks.

hengxin

[Andrew Helwer]

This language construct of binding tuple values to variables is a neat little trick that I had no idea existed in TLA+ before reading the grammar a few weeks back!