rontolisp:catch

(rontolisp:catch future handler)

Returns a fresh future that, on the input's error settlement, invokes handler on the condition and settles to the handler's return value; on successful settlement the value passes through unchanged. If the handler itself signals, the returned future carries THAT condition.

Use rontolisp:catch when the future crosses a boundary as a value and you want a JavaScript .catch-style single-handler fallback. Type-dispatch already exists lexically as (handler-case (rontolisp:await f) (my-err (c) ...)) -- write that instead when the future is right there in the body. A user who wants typed dispatch inside a catch handler writes it explicitly:

(rontolisp:catch f (lambda (c)
                     (handler-case (signal c)
                       (my-err (e) ...)
                       (error (e) ...))))

A non-future first argument is a type-error.

Name collision with cl:catch

Common Lisp's catch / throw is a tag-based non-local exit special form. This operator is rontolisp:catch, in a different package: qualified names never collide (cl:catch still names the CL special form). A user in cl-user (or in a package that :uses both) needs the explicit rontolisp: / rl: prefix to get this operator, and the explicit cl: prefix (or an unqualified bare name in cl-user) to get the tag-based special form. Inside (in-package :rontolisp) a bare catch is neither: the name belongs to cl, which that package does not :use, so it is an "Undefined symbol: CATCH (use CL:CATCH)" error until you qualify it.

Backend support

Same as rontolisp:then: interpreter, JVM, WASM --component. Preview 1 WASM supports the success passthrough only (the error path there needs the futured error-at-await contract that the component backend provides). --no-gc rejects at compile time.