# ApplicativeError and MonadError

## Applicative Error

### Description

`ApplicativeError`

extends `Applicative`

to provide handling for types
that represent the quality of an exception or an error, for example, `Either[E, A]`

### TypeClass Definition

`ApplicativeError`

is defined by the following `trait`

```
trait ApplicativeError[F[_], E] extends Applicative[F] {
def raiseError[A](e: E): F[A]
def handleErrorWith[A](fa: F[A])(f: E => F[A]): F[A]
def handleError[A](fa: F[A])(f: E => A): F[A]
def attempt[A](fa: F[A]): F[Either[E, A]]
//More functions elided
}
```

### Use Case

`Either`

We can start with a less abstract way of performing a function. Here we will divide one number by another.

```
def attemptDivide(x: Int, y: Int): Either[String, Int] = {
if (y == 0) Left("divisor is zero")
else {
Right(x / y)
}
}
```

While fine in the above approach, we can abstract the `Either`

away
to support any other kind of “error” type without having to
create multiple functions with different “container” types.

```
import cats._
import cats.implicits._
def attemptDivideApplicativeError[F[_]](x: Int, y: Int)(implicit ae: ApplicativeError[F, String]): F[Int] = {
if (y == 0) ae.raiseError("divisor is error")
else {
ae.pure(x/y)
}
}
```

The above method summons `ApplicativeError`

to provide behavior representing
an error where the end-user, based on type, will get their
appropriate response.

`ApplicativeError`

is an `Applicative`

, which means all `Applicative`

functions are available for use. One such method is `pure`

, which will
return the `F[_]`

representation, where `F`

could represent `Either`

.

Another method that you will see is `raiseError`

, which will generate
the specific error type depending on what `F[_]`

represents.
If `F[_]`

is an `Either`

, then `ae.raiseError`

will return `Left`

.
If `F[_]`

represents a `Validation`

, then `ae.raiseError`

will return `Invalid`

.
For example, if we want to use an `Either`

as our error representation,
we can do the following:

```
type OnError[A] = Either[String, A]
val e: OnError[Int] = attemptDivideApplicativeError(30, 10)
```

or simply via assignment

```
val f: Either[String, Int] = attemptDivideApplicativeError(30, 10)
```

`Validated`

Given the same function `attemptDivideApplicativeError`

, we can call that
function again but with a different return type, since the `ApplicativeError`

can support other “error” based types. Here we will use `cats.data.Validated`

when calling `attemptDivideApplicativeError`

. Notice that
`attemptDivideApplicativeError`

is the same as we defined above,
so we make no other changes.

```
import cats.implicits._
import cats.data.Validated
type MyValidated[A] = Validated[String, A]
val g = attemptDivideApplicativeError[MyValidated](30, 10)
```

We can inline the right projection type alias, `MyValidated`

, doing the following:

```
val h = attemptDivideApplicativeError[({ type T[A] = Validated[String, A]})#T](30, 10)
```

Or we can use KindProjector to make this more refined and readable

```
val j = attemptDivideApplicativeError[Validated[String, *]](30, 10)
```

### It is an `Applicative`

after all

As a Reminder, this is an `Applicative`

so all the methods of `Applicative`

are available to you to use in
manipulating your values, `ap`

, `mapN`

, etc. In the following example, notice
we are using `Applicative`

’s `map2`

, and of course, `pure`

which also is a
form of `Applicative`

.

```
import cats.implicits._
def attemptDivideApplicativeErrorWithMap2[F[_]](x: Int, y: Int)(implicit ae: ApplicativeError[F, String]): F[_] = {
if (y == 0) ae.raiseError("divisor is error")
else {
val fa = ae.pure(x)
val fb = ae.pure(y)
ae.map2(fa, fb)(_ / _)
}
}
```

### Handling Errors

`ApplicativeError`

has methods to handle what to do when `F[_]`

represents an error.
In the following example, `attemptDivideApplicativeErrorAbove2`

creates an error representation if the divisor
is `0`

or `1`

with the message “Bad Math” or “Waste of Time”.

We will feed the result from `attemptDivideApplicativeErrorAbove2`

into the `handler`

method, where this method will pattern match on the message
and provide an alternative outcome.

```
import cats.implicits._
def attemptDivideApplicativeErrorAbove2[F[_]](x: Int, y: Int)(implicit ae: ApplicativeError[F, String]): F[Int] =
if (y == 0) ae.raiseError("Bad Math")
else if (y == 1) ae.raiseError("Waste of Time")
else ae.pure(x / y)
def handler[F[_]](f: F[Int])(implicit ae: ApplicativeError[F, String]): F[Int] = {
ae.handleError(f) {
case "Bad Math" => -1
case "Waste of Time" => -2
case _ => -3
}
}
```

Running the following will result in `Right(-1)`

```
handler(attemptDivideApplicativeErrorAbove2(3, 0))
```

`handleErrorWith`

is nearly the same as `handleError`

but
instead of returning a value `A`

, we will return `F[_]`

. This could provide us
the opportunity to make it very abstract and return a value from a `Monoid.empty`

.

```
def handlerErrorWith[F[_], M[_], A](f: F[A])(implicit F: ApplicativeError[F, String], M:Monoid[A]): F[A] = {
F.handleErrorWith(f)(_ => F.pure(M.empty))
}
```

Running the following will result in `Right(0)`

```
handlerErrorWith(attemptDivideApplicativeErrorAbove2(3, 0))
```

## MonadError

### Description

Since a `Monad`

extends an `Applicative`

, there is naturally a `MonadError`

that
will extend the functionality of the `ApplicativeError`

to provide `flatMap`

composition.

### TypeClass Definition

The Definition for `MonadError`

extends `Monad`

which provides the
methods, `flatMap`

, `whileM_`

. `MonadError`

also provides error
handling methods like `ensure`

, `ensureOr`

, `adaptError`

, `rethrow`

.

```
trait MonadError[F[_], E] extends ApplicativeError[F, E] with Monad[F] {
def ensure[A](fa: F[A])(error: => E)(predicate: A => Boolean): F[A]
def ensureOr[A](fa: F[A])(error: A => E)(predicate: A => Boolean): F[A]
def adaptError[A](fa: F[A])(pf: PartialFunction[E, E]): F[A]
def rethrow[A, EE <: E](fa: F[Either[EE, A]]): F[A]
}
```

### Use Case

Given a method that accepts a tuple of coordinates, it finds the closest city. For this example we will hard-code “Minneapolis, MN,” but you can imagine for the sake of In this example, you would either consult a database or a web service.

```
def getCityClosestToCoordinate[F[_]](x: (Int, Int))(implicit ae: ApplicativeError[F, String]): F[String] = {
ae.pure("Minneapolis, MN")
}
```

Next, let’s follow up with another method, `getTemperatureByCity`

, that given a
city, possibly a city that was just discovered by its coordinates, we get
the temperature for that city. Here, for the sake of demonstration,
we are hardcoding a temperature of 78°F.

```
def getTemperatureByCity[F[_]](city: String)(implicit ae: ApplicativeError[F, String]): F[Int] = {
ae.pure(78)
}
```

With the methods that we will compose in place let’s create a method that will
compose the above methods using a for comprehension which
interprets to a `flatMap`

-`map`

combination.

`getTemperatureFromByCoordinates`

parameterized type
`[F[_]:MonadError[*[_], String]`

injects `F[_]`

into `MonadError[*[_], String]`

thus if the “error type” you wish to use is `Either[String, *]`

, the `Either`

would be placed in the hole of `MonadError`

, in this case,
`MonadError[Either[String, *], String]`

`getTemperatureFromByCoordinates`

accepts a `Tuple2`

of `Int`

and `Int`

, and we
return `F`

which represents our `MonadError`

which can be a type like `Either`

or
`Validated`

. In the method, since either `getCityClosestToCoordinate`

and
`getTemperatureByCity`

both return potential error types and they are monadic we can
compose them with a for comprehension.

```
def getTemperatureByCoordinates[F[_]: MonadError[*[_], String]](x: (Int, Int)): F[Int] = {
for { c <- getCityClosestToCoordinate[F](x)
t <- getTemperatureByCity[F](c) } yield t
}
```

Invoking `getTemperatureByCoordinates`

we can call it with the following sample,
which will return `78`

.

NOTE: infix `->`

creates a `Tuple2`

. `1 -> "Bob"`

is the same as `(1, "Bob")`

```
type MyEither[A] = Either[String, A]
getTemperatureByCoordinates[MyEither](44 -> 93)
```

With TypeLevel Cats, how you structure your methods is up to you, if you wanted to
create `getTemperatureByCoordinates`

without a Scala
context bound for `MonadError`

,
but create an `implicit`

parameter for your `MonadError`

you can have access to some
additional methods.

In the following example, we create an `implicit`

`MonadError`

parameter
and call it `me`

. Using the `me`

reference, we can call any one of its
specialized methods, like `raiseError`

, to raise an error representation
when things go wrong.

```
def getTemperatureFromByCoordinatesAlternate[F[_]](x: (Int, Int))(implicit me: MonadError[F, String]): F[Int] = {
if (x._1 < 0 || x._2 < 0) me.raiseError("Invalid Coordinates")
for { c <- getCityClosestToCoordinate[F](x)
t <- getTemperatureByCity[F](c) } yield t
}
```