>
> is there a way round it?
>
Something else I need to point out is that you really should not use a Set
for order. Sets are specifically unordered, and provide no guarantees of
order. Try calling first, second and last on #{1 2 3} for example, you'll
probably get things back in a different order.
If it happens to work, it is accidental, and should not be relied on. If
you want order, use a Vector or a List. Vectors are probably going to feel
more natural, since they append to the end.
You use a Set over a Vector most of the time, because you are specifically
telling the consumers of the collection not to rely on order, that the
order is meaningless and no logic should be derived from it. Similarly, if
you use a Vector over a Set it should be because the order is guaranteed
and does matter, and consumers of the collection are welcomed to assume
this and build logic that is order dependent.
That said, yes there is a way around it, you can call persistent! to get
back a PersistentHashSet, and then call first on it. Going from a transient
to a persistent is O(1), so its really fast. You can keep alternating
between persistent and transient as your needs change between reading and
iterating back to updating.
Reading and iterating over a persistent is just as fast, the transient
provides no value. Transient will only make updates and inserts faster.
On Saturday, 24 February 2018 07:15:02 UTC-8, Alan Forrester wrote:
>
> Calling (first #{1}) gives the result 1.
>
> Calling (first (transient #{1})) gives an error:
>
> “IllegalArgumentException Don't know how to create ISeq from:
> clojure.lang.PersistentHashSet$TransientHashSet clojure.lang.RT.seqFrom
> (RT.java:550)”
>
> Is this behaviour intended?
>
> And whether or not this is intentional is there a way round it?
>
> Alan Forrester
>
>
>
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