peek() returns a tuple instead of a key

[vixsomnis]

Arch Linux Julia package:
Julia Version 0.3.10
Commit c8ceeef (2015-06-24 13:54 UTC)

(I believe this issue is also present in the latest commit and docs.)

Instead of returning only the key, peek() returns a tuple consisting of a (k,v) tuple.

Is the documentation wrong or is the behavior wrong?

[ivarne]

The return type was just changed from Tuple to Pair in #12327, so I assume this should be considered a documentation issue.

[vixsomnis]

A pair is still essentially two values, however -- which is the conflict I am referencing. Should peek() return a single value in the same way as dequeue!()?

[JeffBezanson]

@dcjones @kmsquire Was the intent to return just the key or the pair?

[kmsquire]

When @dcjones created it in 39b5c3d#diff-10b4d49367d18d7903690d5b40f848fcR184, it only returned a key.

@timholy changed it (back?) to a pair in #8011, and that seems to be the intent. I also believe he uses this code, so he should probably chime in.

(I don't think I ever touched this code.)

[timholy]

Small correction: it always returned the (K,V) tuple. In @dcjones version, xs was an array of tuples, so returning the first element returned a (K,V) pair. For performance reasons (back before tuples were really fast), I changed xs to an array of pairs, and rewrote peek return a tuple for backwards compatibility.

[vixsomnis]

This is really more about whether peek should return a single value (like how dequeue! returns just the key) or both the key and the value associated with it.

As it stands right now, peek has no real mutating alternative. dequeue! only returns the key, but you can't get the value for that after dequeuing since the priority queue no longer contains the key.

k = dequeue!(pq)
v = pq[k] # doesn't work
dostuff

We can work around this with peek.

kv = peek(pq)
dostuff
dequeue!(pq) # but we don't need the return value anymore

The intent of the second example is obscured by the use of peek (non-mutating) and throwing away the return value of dequeue!.

Should peek return only K? Should dequeue! return (K,V)?

Another possibility is to give pop! a method to handle priority queues that returns a (K,V) the same way peek does, thus keeping dequeue! consistent with enqueue!.

[KristofferC]

I think this can be closed since peek and corresponding data structure has been moved to https://github.com/JuliaCollections/DataStructures.jl?