Errors

Errors in Bridje are anomalies: tags, in brj.core, over a record of details. The categories follow cognitect.anomalies — each says what kind of failure it is, and so what a caller might do about it.

Categories

Unavailable

The callee isn’t available — try again, maybe later.

Interrupted

The operation was interrupted — brj.concurrent’s interruption throws this.

Busy

The callee is overloaded — back off and try again.

Incorrect

The caller made a mistake — fix the call.

Forbidden

The caller isn’t allowed — fix the permissions.

Unsupported

The operation isn’t supported — don’t retry.

NotFound

The thing asked for doesn’t exist.

Conflict

The request conflicts with the current state — coordinate, then retry.

Fault

The callee has a bug — nothing the caller can do.

Host

An exception thrown by the host — a Java exception.

Throwing

throw(anomaly)

Throws anomaly. throw’s return type is Nothing, so it fits anywhere a value is expected.

throw(NotFound{.exnMessage "no such user"})

An anomaly’s record can carry any keys; .exnMessage, a Str, is the conventional one, and becomes the exception’s message on the host.

Catching

try: body… catch: pattern handler … finally: cleanup…

Evaluates the body; if it throws, the first catch: pattern that matches the anomaly selects the handler. Patterns are as in case:, and a final unpaired form is the default. A pattern’s tag must be an anomaly — Thing is not an anomaly, so a catch cannot match it — and a lowercase catch-all binds an Anomaly. The try: as a whole is the join of its body and its handlers; finally: contributes nothing to its type.

try:
  findUser(id)
  catch: NotFound(e) nil
  catch: Host(e) throw(Fault{.exnMessage "lookup failed"})
  finally: closeConnection()

An anomaly’s keys are all optional, so a caught anomaly’s message is read with .?exnMessage(e); destructuring it, Fault{exnMessage}, is an error. An anomaly that no pattern matches carries on up the stack.