How to get a handle on the binary operator functions like `==`, `>`, etc.? For "normal" functions...

# getting-started

[David Kubecka]

How to get a handle on the binary operator functions like ==, >, etc.? For "normal" functions I would use :: but not sure how to do that for these operators.

I want to convert user input (as string) to the operator function and then call this function on the provided values.

[Landry Norris]

All operators have a function name, so you can use that. 1::plus works if you specify which plus. It does require that the operator is defined for the type.

[Riccardo Lippolis]

== -> equals
> -> compareTo

[Landry Norris]

See here for list of names

[Riccardo Lippolis]

(tip: if you're using IntelliJ, you can use the go to implementation feature to look this up via your IDE)

[David Kubecka]

I already tried ::equals but that gives me only a single parameter function.

[Luke Armitage]

::equals is a single argument function - it compares the receiver to the argument. 😄

[Riccardo Lippolis]

val instanceEquals: KFunction1<Any?, Boolean> = "My string"::equals

val stringEquals: KFunction2<String, Any?, Boolean> = String::equals

[Landry Norris]

T.equals(Any) is the general signature for the equals method, where in this case T means a type.

[David Kubecka]

So to obtain a boolean function for comparison I probably can't use simple reflection ::, right? Instead, I would have to do something like { x: T, y: T -> x.compareTo(y) > 0 }

[Landry Norris]

You can use x::compareTo to get the method.

[David Kubecka]

yeah, but that doesn't return boolean

[Landry Norris]

I see. You’d have to define one or use the lambda you provided.

[Landry Norris]

You could define T.compareToAsBoolean(y: T) -> Boolean, but with better #naming

[David Kubecka]

Hmm, but thanks to this intended usage of comparison the end result is quite verbose, e.g.

val valueCompareTo = it.value.toBigDecimal()::compareTo
            return when (it.operator) {
                FilterOperator.EQUAL -> { money: MoneyDto -> valueCompareTo(money.amount) == 0 }
                FilterOperator.GREATER_THAN -> { money: MoneyDto -> valueCompareTo(money.amount) > 0 }
                FilterOperator.GREATER_THAN_OR_EQUAL -> { money: MoneyDto -> valueCompareTo(money.amount) >= 0 }
                FilterOperator.LESS_THAN -> { money: MoneyDto -> valueCompareTo(money.amount) < 0 }
                FilterOperator.LESS_THAN_OR_EQUAL -> { money: MoneyDto -> valueCompareTo(money.amount) <= 0 }
                else -> error("operator ${it.operator} not supported here")
            }

Ideally, I would like to return only the operator from the when expression and inject that into the money lambda (defined at single place only), but since there's no such thing as a comparison function that returns boolean I have to repeat that code. Or... ?

[Riccardo Lippolis]

a function like isGreaterThan needs to be defined somewhere, in that case. You can choose to put it in the when expression, in the MoneyDto (if you have control over it), or define it as an extension function, for example

[David Kubecka]

I see. My original thoughts perhaps were that this function surely must already be defined somewhere because

fun <T> isGreaterThan(a: T, b: T) = a.compareTo(b) > 0

seems to be quite generic 🙂

[David Kubecka]

Oh, this apparently can't be defined in a generic way 😞

[Landry Norris]

What do you mean?

[Landry Norris]

fun <T: Comparable<T>> isGreaterThan(a: T, b: T) = a.compareTo(b) > 0

[Landry Norris]

You didn’t specify that T has to be comparable.

[David Kubecka]

Anyway, isn't it reasonable to expect these functions to be in the stdlib?

[Landry Norris]

I’d say no. The stdlib is included with all projects, so adding something should be an important change. The current Int behavior is consistent with the Java Comparable that exists, and provides more information (can tell you how much bigger or smaller if implemented that way).

[Landry Norris]

The size of the stdlib is currently a major blocker for K/N on embedded systems. The stdlib alone creates a binary too big for most embedded systems.

[David Kubecka]

I don't expect the current operators to be replaced. Just that those generic binary functions are added exactly for cases like mine.

[David Kubecka]

I hear the binary size argument, although I'm not sure how those six small functions compare with the quite rich collections class (which of course I'm happy to use in my BE projects)

[Landry Norris]

You can create a proposal for it to be added, but do keep in mind that you may only need 6 methods, but someone else may have another few methods that make sense for them, and someone else has another few methods they want.

[Landry Norris]

I’d imagine somewhere there’s a third party library that brings in these useful helpers (kind of like arrow with functional methods). This keeps the stdlib small, while allowing methods to be added easily.

[David Kubecka]

Anyway, I'm still struggling even to use these binary functions 😞 My hope was something like this:

val filteredValue = it.value.toBigDecimal()
val cmp = filteredValue::isEqualTo

Apparently, that's not how it's supposed to be...

[Landry Norris]

If you want to use foo::isEqualTo , you have to make sure the signature is T.isEqualTo(other: T) -> Boolean . If it’s like your greater than method above, you defined isEqualTo(x: T, y: T) -> Boolean , which is different.

[David Kubecka]

I see now, of course...

[Landry Norris]

Without the receiver, it’s just ::isEqualTo

[David Kubecka]

I originally wanted to use just ::isEqualTo and assign it to val but that's not possible without specifying the T. Will consider the other option (extension functions)

[David Kubecka]

I guess there's no shorthand syntax for partial function application in Kotlin.

[Landry Norris]

I’m sure if you specified a type for the val, it could get it.

[David Kubecka]

Sure ^, I meant if there's a nice (without writing the full lambda) way how to get from two arg isEqualTo to single arg isEqualTo (with fixed first arg).