Is there any requirement on implementing `Raise` ?...

# arrow

[CLOVIS]

Is there any requirement on implementing Raise ? It's really tempting to create a

object TestRaise : Raise<Any> {
    override suspend fun <B> raise(r: Any): Nothing {
        throw AssertionError("Unexpected result from $r")
    }
}

to be able to write tests like

runTest {
    val result = expensiveOperation().bind()
    result shouldBe 42
}

instead of

runTest {
    val result = expensiveOperation()
    result shouldBe 42.right()
}

[gpopides]

can you make use of the arrow assertions?

result.shouldBeRight(42)

[CLOVIS]

It's more convenient to failfast than to use an either result. It also works with non-either types that use the Raise DSL (which the arrow assertions cannot manage to my knowledge)

[Youssef Shoaib [MOD]]

Why not just have runTest be:

inline fun runTest(block: Raise<Any>().() -> Unit) {
  either {
    block().mapLeft { throw AssertionError("Unexpected result from $it") }
  }
}

[Youssef Shoaib [MOD]]

In general, there's not really a reason to implement Raise , and the only reason it's an interface is because of NullableRaise, OptionRaise, etc, which will disappear when context receivers are stable

[simon.vergauwen]

I would advise against it. The suggestion that @Youssef Shoaib [MOD] made is a much better solution, and there is many different styles DSLs you could come-up with.

[P A]

As another example, we use a similar pattern to Youssef’s. Our code looks like the screenshot below where a “respond” function takes care of handling the either.

[simon.vergauwen]

@P A really neat use of context receivers, and Arrow with Spring 😍

[CLOVIS]

@Youssef Shoaib [MOD] but then the assertion's stack trace doesn't point to the location of the error, does it?

@P A I don't understand your point, the question was about tests?

[Youssef Shoaib [MOD]]

Use the new tracing API that got merged into arrow recently (not sure if it's out yet in a version or not, so you might need to wait). It allows you to access the stack trace of the line that called raise

[Youssef Shoaib [MOD]]

I believe P A's point was that the respond function does something similar internally where it takes care of throwing an exception (likely one with an HTTP status code or something) in case of a raised value, so it's a similar pattern as what you're doing for tests

[CLOVIS]

@Youssef Shoaib [MOD] Oh, I forgot about it! Then, I'll keep my Raise receiver in tests, keep the implementation that throws an AssertionError for now, and replace it with the tracing API whenever that reaches my projects.

[simon.vergauwen]

The traces API has been released in 1.2.0-RC. I slapped OptIn on it though 😅
So please test and use it, so we can remove it before 2.0 or refine the API first.

[CLOVIS]

I don't see anything in the docs about it, how should I use it?

[Youssef Shoaib [MOD]]

https://github.com/arrow-kt/arrow/blob/main/arrow-libs/core/arrow-core/src/jvmTest/kotlin/arrow/core/raise/TraceJvmSpec.kt#L7:

either<RuntimeException, Nothing> {
      traced({ raise(RuntimeException("")) }) { traced, raised ->
        // Remove first 2 lines:
        // arrow.core.raise.RaiseCancellationException
        //	at arrow.core.raise.DefaultRaise.raise(Fold.kt:187)
        val trace = traced.stackTraceToString().lines().drop(2)

        // Remove first line:
        // java.lang.RuntimeException:
        val exceptionTrace = raised.stackTraceToString().lines().drop(1)

        trace shouldBe exceptionTrace
      }
    }

Shows how to get the stack trace out of it.