The Failure Type

One of the types you'll want to familiarize yourself with is Unison'sFailuretype. You'll see this type in the signatures of some of the functions in thebaselibrary, for examplecatch : '{g, Exception} a ->{g} Either Failure a.Failureis a unique type whose purpose is to capture information about an error.

Let's look at what we need to construct a failure.

Failure : Type -> Text -> Any -> Failure

As an example, constructing aFailureupon attempting to save a duplicateUserdatabase error might look like

failure.example1 : Failure

failure.example1 : Failure
failure.example1 =
  Failure
    (typeLink DatabaseError)
    "The username column already contains a value for entry: Bob"
    (Any (User "Bob"))

The first argument to thedata constructorforFailureis aType.

You'll need to create aTypewith thetypeLinkliteral. The typelink literal allows us to represent a Unison type as a first class value.

Commonly, the type that we're linking to is some data type which represents the domain of the error, for exampleDatabaseErrororGeneric.

The second argument toFailureis aTextbody which should be a descriptive error message.

The third argument is of typeAny.Anyis another Unison built-in. It wraps any unison value. For example:

Any 42
⧨
Any 42

or

Any [1, 2, 3]
⧨
Any [1, 2, 3]

You can useAnyto capture the value that has produced the error result. If that value is not useful for error reporting, you can always useUnitas a placeholder,Any ().

📚Unison provides a helpfulGenericfailure constructor,Generic.failurefor when you don't have a particular type that models your error domain. It takes in an error message and a value of any type to produce aFailure:Generic.failure : Text -> a -> Failure

genericFailure : Failure

genericFailure : Failure
genericFailure =
  Generic.failure
    "A failure occurred when accessing the user" (User "Ada")