# Re: [tlaplus] new to TLA: difference between if-then-else and logic or

Thanks Stephan. After I replace <=> to =, it works.

Regards,
Yong

On Sunday, February 18, 2018 at 11:59:43 PM UTC-8, Stephan Merz wrote:
"<=>" expects Boolean arguments and denotes equivalence, you want to write "=" instead.

Regards,
Stephan

> On 19 Feb 2018, at 07:31, Yong Liu <liu...@xxxxxxxxx> wrote:
>
> Hi all,
>
> I'm new to TLA+.
>
> I tried the HourClock example, and found it will fail after I replace:
> HCnxt == hr' = IF hr # 12 THEN hr + 1 ELSE 1
> to
> HCnxt == \/ hr # 12      /\ hr' = hr + 1
>                 \/ hr <=> 12 /\ hr' = 1
> I think above statements are equivalent, if not, please kindly let me know the difference.
>
> Below is my HourClock for your reference.
>
> ----------------------------- MODULE HourClock -----------------------------
>
> EXTENDS Naturals
> VARIABLE hr
>
> HCini == hr \in 1..12
>
> \*HCnxt == hr' = IF hr # 12 THEN hr + 1 ELSE 1
>
> HCnxt == \/ hr # 12   /\ hr' = hr + 1
>         \/ hr <=> 12 /\ hr' = 1
>
> HC == HCini /\ [][HCnxt]_hr
>
> -----------------------------------------------------------------------------
> THEOREM HC
>
> =============================================================================
>
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