Hello,
your function does not follow the contract of fold. What you provided is
basically the reduce function, but it doesn't work together with fold. For
fold you need to merge the different maps you created in the subtasks. So
the combine function must look different.
(defn my-frequencies
[coll]
(r/fold
(fn
([] {})
([a b] (merge-with + a b)))
#(update-in %1 [%2] (fnil inc 0))
{}
coll))
Compare this example. The contract for the combine function requires that
the combine function may be called with zero arguments to return the
seeding value for the subtask. In our case an empty map. (This is were your
example blows up.) To combine two maps we use merge-with. The update-in is
basically the reduce function. The reducers library provides the monoid
helper to ease the pain a little. (Although it doesn't buy us much in your
example.)
(defn my-frequencies
[coll]
(r/fold
(r/monoid
#(merge-with + %1 %2)
(constantly {}))
#(update-in %1 [%2] (fnil inc 0))
{}
coll))
Additionally you cannot use transients at the moment with fold.
Kind regards
Meikel
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