While working of #1570 I tried this to verify that a constant key results in no change to the order of the inputs:

> order [1 3 2 5 4] &key={|a| put $true }
Exception: bad value: inputs to "compare" or "order" must be comparable values, but is uncomparable values

I thought I had a bug in my &key implementation but it turns out that the current Elvish implementation does not support comparing booleans:

> order [$true $false]
Exception: bad value: inputs to "compare" or "order" must be comparable values, but is uncomparable values

Yes, this is an odd thing to do but it should be possible to compare booleans. Note that this isn't a problem with the order command -- it's a problem with the compare command (whose implementation order uses for comparisons):

> compare $true $false
Exception: bad value: inputs to "compare" or "order" must be comparable values, but is uncomparable values
[tty 25], line 1: compare $true $false