As a little note here today, here’s an example of how to use a Scala Either
inside a for
-expression when using ZIO 2. In the code you’ll see that I convert each Either
into a ZIO
in each line of the for
-expression.
Here’s the Scala/ZIO code:
//> using scala "3"
//> using lib "dev.zio::zio::2.1.0"
package zio_either_test
import zio.*
import zio.Console.*
import java.io.IOException
import scala.util.control.Exception.*
def makeInt(s: String): Either[Throwable, Int] =
allCatch.either(s.toInt)
object ZioEitherInForExpr extends ZIOAppDefault:
val blueprint: ZIO[Any, Throwable, Int] =
for
a <- ZIO.fromEither(makeInt("1"))
b <- ZIO.fromEither(makeInt("uh oh"))
c <- ZIO.fromEither(makeInt("3"))
yield
a + b + c
val run = blueprint.foldZIO(
failure => printLineError(s"FAILURE = $failure"),
success => printLine( s"SUCCESS = $success")
)
Notice that makeInt
returns an Either
, but I want to use ZIO
values inside the for
-expression, so I convert each Either
into a ZIO
with ZIO.fromEither
.
When the string in the second line of the for
-expression is "uh oh"
, I see this output when I run this ZIO application:
$ scala-cli ZioEitherForExpression.scala
FAILURE = java.lang.NumberFormatException: For input string: "uh oh"
As I have noted in other articles and videos, that’s because the for
-expression short-circuits on that line of code.
But if I change that "uh oh"
to a "2"
, I see this output instead:
$ scala-cli ZioEitherForExpression.scala
SUCCESS = 6
A better ZIO/Either example
One other thing I have should have shown in this example is that the only reason you’d convert these Either
values into ZIO
values in this specific example is if you wanted to use other ZIO
values in the for
expression, like this:
for a <- ZIO.fromEither(makeInt("1")) b <- ZIO.fromEither(makeInt("uh oh")) c <- ZIO.fromEither(makeInt("3")) d <- ZIO.succeed(42) // some other ZIO value here yield a + b + c
If you don’t need to mix another ZIO
value into the for
-expression, Either
values work just fine in for
-expressions by themselves:
for a <- makeInt("1") b <- makeInt("uh oh") c <- makeInt("3") yield a + b + c
In this example, each makeInt
call returns an Either
value, and this works (as I have shown in other examples on this website).
Summary: ZIO, Either, and for-expressions
In summary, if you wanted to see how to use ZIO 2 along with Either
and ZIO
values in a Scala for
-expression, I hope this little example is helpful. Also note that the same technique can be used with Scala Option
and Try
values, almost exactly as shown with the Either
values here.
Also, for more information on this topic, see my free video on How the Scala Either type relates to the ZIO type.
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