likern - 4 months ago 18

Scala Question

I have a unit test, which test some solution. But this test code can also be applied for testing the other, very similar solution. What I want to make is code of test be generic to be applied to both solutions, like this:

`describe("when table contains all correct rows") {`

it("should be empty") {

def check[T](func: T => List[Row]) = {

val tableGen = new TableGenerator()

val table: Vector[Row] = tableGen.randomTable(100)

.sortWith(_.time isBefore _.time).distinct

val result: List[Row] = func(table)

assert(result.isEmpty)

}

check(Solution.solution1)

check(Solution.solution2)

}

}

where solutions have types:

`solution1: IndexedSeq[Row] => List[Row]`

solution2: Seq[Row] => List[Row]

how

And what's the best approaches to write this (might be in other way) with eliminated code duplication?

When I try to compile this code I get type mismatch error in

`func(table)`

`Error:(36, 29) type mismatch;`

found : table.type (with underlying type scala.collection.immutable.Vector[com.vmalov.tinkoff.Row])

required: T

val result = func(table)

Answer

For this to work, you need to be able to pass a `Vector[Row]`

to `func`

, so any `Vector[Row]`

has to be a `T`

; that is, `T`

is a supertype of `Vector[Row]`

. You can tell this to the compiler by using a type parameter bound:

```
def check[T >: Vector[Row]](func: T => List[Row])
```

Alternately, by the above reasoning, a function `T => List[Row]`

will also be a function `Vector[Row] => List[Row]`

precisely when `T`

is a supertype of `Vector[Row]`

, and the Scala compiler knows about this (functions are *contravariant* in their argument type(s)). So this signature is equivalent to simpler

```
def check(func: Vector[Row] => List[Row])
```

Of course, you can generalize this, but how much depends on your specific desires. E.g. you can replace `List[Row]`

with `Seq[Row]`

(everywhere), or with a type parameter and pass an extra function to `check`

:

```
def check[A](func: Vector[Row] => A)(test: A => Boolean) = {
val table = ...
val result = func(table)
assert(test(result))
}
check(Solution.solution1)(_.isEmpty) // the compiler infers A is List[Row]
```