I have a task to transform the following code-block:
val instance = instanceFactory.create
val result = instance.ackForResult
to for-comprehension expression.
As for-comprehension leans on enumeration of elements, I tried to get around it with wrapper class:
case class InstanceFactoryWrapper(value:InstanceFactory) {
def map(f: InstanceFactory => Instance): Instance
= value.create()
}
where map-method must handle only one element and return a single result: Instance
I tested this approach with this expression:
for {
mediationApi <- InstanceFactoryWrapper(instanceFactoryWrapper)
}
But it does't work: IDEA recommends me to use foreach in this part. But "foreach" doesn't return anything, as opposed to map.
What am I doing wrong?
Simply put when working with List\Option\Either or other lang types comprehensions are useful to transform nested map\flatMap\withFilter into sequences.
But what about your own classes or other 3rd party ones?
You need to implement monadic operations in order to use them in for-comprehensions.
The bare minimum: map and flatMap.
Take the following example with a custom Config class:
case class Config[T](content: T) {
def flatMap[S](f: T => Config[S]): Config[S] =
f(content)
def map[S](f: T => S): Config[S] =
this.copy(content = f(content))
}
for {
first <- Config("..")
_ = println("Going through a test")
second <- Config(first + "..")
third <- Config(second + "..")
} yield third
This is how you enable for-comprehension.