[variance question] I'm building a DSL that repres...

# getting-started

[CLOVIS]

[variance question] I'm building a DSL that represents "expressions". Expressions are typed using two criteria:
• Their return value (same as the Kotlin type of an expression)
• The "context" in which they are available.
This brings a design like:

interface Expression<Context : Any, out Result>

Expressions can be combined together using the typical operators;

val first: Expression<A, Int> = …
val second: Expression<A, Int> = …
val sum: Expression<A, Int> = first + second

The "context" must be the same for both operands for the operation to make sense. So far, this is simple to do.

The difficulty is the existence of a special class of values that simply ignores context. That is, if put in an operation where the other operand has a context x , this special value behaves as if its context was also x, and thus the result expression should have context x . Essentially, this special value is "contextless", a kind of joker.

What is the correct variance to represent this special value?

[CLOVIS]

Said otherwise, the context rule "flows" opposite to the type rule;
• The return type of two operands decides the return type of an expression,
• The context of the expected result of an expression dictates the acceptable contexts of the operands

[Daniel Pitts]

So, is it basically the "Any" context?

[Daniel Pitts]

interface Expression<in Context : Any, out Result>

operator fun <Context : Any, Result> Expression<Context, Result>.plus(other: Expression<Context, Result>)
    : Expression<Context, Result> =  TODO()

data class Contextless<T>(val value: T) : Expression<Any, T>

fun <C:Any> foo(exp: Expression<C, Int>) {
    val sum: Expression<C, Int> = exp + Contextless(1)
}

Does this work for your case?

[CLOVIS]

Yep, that was the solution, thanks!

[CLOVIS]

I was confused but yeah, in Context : Any was the solution