Simple Generic Function in Kotlin Fails

Question

Here's a simple generic function in Kotlin:

fun <T> twice(x: T) : T { return 2 * x }

Attempting to build this (either in a project or REPL) results in the following errors:

error: none of the following functions can be called with the arguments supplied: 
public final operator fun times(other: Byte): Int defined in kotlin.Int
public final operator fun times(other: Double): Double defined in kotlin.Int
public final operator fun times(other: Float): Float defined in kotlin.Int
public final operator fun times(other: Int): Int defined in kotlin.Int
public final operator fun times(other: Long): Long defined in kotlin.Int
public final operator fun times(other: Short): Int defined in kotlin.Int
fun <T> twice(x: T) : T { return 2 * x }
                                   ^

If I switch the return statement operands to x * 2, the error messages become:

error: unresolved reference. None of the following candidates is applicable because of receiver type mismatch: 
@InlineOnly public inline operator fun BigDecimal.times(other: BigDecimal): BigDecimal defined in kotlin
@InlineOnly public inline operator fun BigInteger.times(other: BigInteger): BigInteger defined in kotlin
fun <T> twice(x: T) : T { return x * 2 }
                                   ^

What am I missing here?

s1m0nw1 · Accepted Answer · 2019-06-12 05:12:05Z

3

Since T could be anything, the compiler is not able to find a matching times operator. As you can see in the error message, for Int, there a multiple alternatives available

public final operator fun times(other: Byte): Int defined in kotlin.Int
public final operator fun times(other: Double): Double defined in kotlin.Int
public final operator fun times(other: Float): Float defined in kotlin.Int
public final operator fun times(other: Int): Int defined in kotlin.Int
public final operator fun times(other: Long): Long defined in kotlin.Int
public final operator fun times(other: Short): Int defined in kotlin.Int

But unfortunately there's no generic times function, which can be used with e.g. Number. I'm afraid in this case, you would have to create an overload for each type you want to be handling, i.e., Double, Int, etc.

answered Jun 12, 2019 at 5:12

s1m0nw1

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1 Comment

Phil Over a year ago

The equivalent template function in C++, template<class T> T twice(T x) { return 2*x; } works as one would expect. So as it currently stands, Kotlin generics cannot do math on the generic type. Interesting.

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