Kotlin Map using Operators

Question

Started playing the weekend with Kotlin and trying to get maps working with operators. Somehow Kotlin tells me I am confusing it with ambiguity.

Here is code that works (syntactically not like I want it to):

var columns = sortedMapOf("a" to 1, "b" to 2)
columns.plusAssign("c" to 3)

And here is code that simply does not compile (but syntactically closer to what I want)

var cs = sortedMapOf(1 to "a", 2 to "b")
cs += Pair(3, "c")

What shorthand operator magic/casting am I missing?

Thanks in advance.

I understand the question, but I would just resolve it with columns.put("c", 3); — JB Nizet
– JB Nizet, Commented Nov 1, 2015 at 7:46
JB, agreed - there is a 100 ways to make it work (functionally). The point is that Kotlin supports the += operator and that it should work, and I want to use it as it reads mathematically I want my code to read. Also, by me not getting it to work means that I am conceptually missing something — user1210708
– user1210708, Commented Nov 1, 2015 at 7:50
I relayed your question on the kotlin slack channel. Let's see if an answer comes up... — JB Nizet
– JB Nizet, Commented Nov 1, 2015 at 7:53
JB - in follow up to your answer of columns.put... even more natural would be code cs[4] = "f" — user1210708
– user1210708, Commented Nov 1, 2015 at 8:29

Ilya · Accepted Answer · 2015-11-01 09:17:00Z

7

The ambiguity here is because Kotlin can interpret the expression cs += Pair(3, "c") either as operation creating new map from the original map and the given pair and assigning that map back to variable cs = cs.plus(Pair(3, "c")), or as operation mutating the original map cs.plusAssign(Pair(3, "c"))

To disambiguate this situation, follow the Kotlin motto — make val, not var!

When you declare cs as val (non-mutable variable), it cannot be reassigned once it has been initialized, so the only operation becomes available here is plusAssign.

answered Nov 1, 2015 at 9:17

Ilya

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4 Comments

JB Nizet Over a year ago

Can you confirm that there's no way to resolve the ambiguity if cs is a var and not a val?

user1210708 Over a year ago

Ilya - thanks - confirmed and working. Wow! Don't quite know what to make of it (definitely is a change of pattern for everything I was used to in pretty much every other language)... but will look out for it now. The other thing is that why can it figure out cs.plusAssign() but not cs.+= with the same Pair parameter. I would naturally have thought that it knew what was coming and where it was going.

user1210708 Over a year ago

Sorry - the second question you answered already... was just thinking out loud.

voddan Over a year ago

Also note that technically there is no problem for a compiler to resolve this issue automatically. But Kotlin reports ambiguity on purpose, just to ease the life of a human reader.

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