Iterating function arguments in Clojure

Question

I am having trouble iterating over the function arguments passed in to the following function.

(defn iterateDates
  [& dates]
  (let [ukFormatter (java.time.format.DateTimeFormatter/ofPattern "dd-MM-yyyy")]
    (for [i [dates]]
      (java.time.LocalDate/parse i ukFormatter))))

(iterateDates "09-10-2019" "10-10-2019" "11-10-2019")

This however when called, returns the following error:

Error printing return value (ClassCastException) at clojure.core/getOldestDate$iter$fn$fn (core.clj:96).
clojure.lang.ArraySeq cannot be cast to java.lang.CharSequence

I am not sure on how to iterate over the arguments passed in and take each element as a separate value which can then be passed to another function.

My goal ultimately with the code will be to compare a list of dates and find the oldest date in there. This code just tries to parse each argument as a date.

This code won't run. You have a bracket mistatch at (for [i [dates)]. Please post the actual code. Also you have the body of the for wrapped in two pairs of parenthesis, which will cause the return of parse to be called as a function. Is that your intent? Also, please post the full stack trace. On first brush, that error doesn't seem to stem from this code. — Carcigenicate
– Carcigenicate, Commented Oct 9, 2019 at 12:14
I have been running this in REPL and that's where I got the error, running it as normal does not produce an error but neither an output. — bluiska
– bluiska, Commented Oct 9, 2019 at 12:32
I think you just mean (for [i dates]. I don't know how that would cause that error, but you're passing the entire dates sequence to parse at once with how you have it now. — Carcigenicate
– Carcigenicate, Commented Oct 9, 2019 at 12:35

jahson · Accepted Answer · 2019-10-09 12:51:53Z

3

(defn iterateDates
  [& dates]
  (let [ukFormatter (java.time.format.DateTimeFormatter/ofPattern "dd-MM-yyyy")]
    (for [i dates]
      (java.time.LocalDate/parse i ukFormatter))))

(iterateDates "09-10-2019" "10-10-2019" "11-10-2019")

This version should work.

You wrote (for [i [dates]] in your original code, and it worked as you unintentionally specified:

It used a vector [] of dates for iteration, where dates is already a sequence.
The first element of this vector is the dates, which is an AraySeq.
java.time.LocalDate/parse tried to parse ArraySeq as a CharSequence and failed.

answered Oct 9, 2019 at 12:51

jahson

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1 Comment

bluiska Over a year ago

Thank you very much for your answer and the explanations. I realise now the issue and the code indeed worked.

bluiska · Accepted Answer · 2019-10-15 09:44:37Z

0

Thought I'd update the answers as I have found another way to achieve this without having to iterate the args:

(defn iterateDates
  [& dates]
  (let [ukFormatter (java.time.format.DateTimeFormatter/ofPattern "dd-MM-yyyy")]
    (map #(java.time.LocalDate/parse % ukFormatter) dates)))

The map function will apply the date formatting to every element of dates and return it as a new list.

answered Oct 15, 2019 at 9:44

bluiska

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