try is a control expression for handling synchronous computations that might fail.

A try expression has 3 parts:

  • statements that returns a Result

  • catch expressions

  • a return expression

Let's see an example where we are using this expression to decode an object:

record User {
email : String,
name : String
module Example {
fun decodeUser (object : Object) : Result(Object.Error, User) {
try {
Try to decode the email from the object,
if succeeds the value is assigned to the
"email" variable.
email =
|> Object.Decode.field("email", Object.Decode.string)
Same for the name.
name =
|> Object.Decode.field("name", Object.Decode.string)
At this point we have the fields so we can return
a result with the user.
email = email
name = name
If any of the decoders fail we handle it here and
return an result with the error.
} catch Object.Error => error {

Keep in mind that you need to handle all possible errors that can be returned from a statement, although the compiler has your back here and will show an error if you forgot one.

In contrast to the do expressions, the value of a try expression is the value of its last expression. So every catch expression must also return the same type.