Exception Matches

OCaml’s try construct is good at dealing with exceptions, but not so good at handling the case where no exception is raised. This feature (now part of OCaml 4.02.0) implements a simple extension to try that adds support for handling the “success” case.

The try construct in current OCaml supports matching against raised exceptions but not against the value produced when no exception is raised. Contrariwise, the match construct supports matching against the value produced when no exception is raised, but does not support matching against raised exceptions. As implemented, the patch addresses this asymmetry, extending match with clauses that specify the “failure continuation”:

match expr with pattern_1 -> expr_1 | ... | pattern_n -> expr_n | exception pattern_1' -> expr_1' | ... | exception pattern_n' -> expr_n'

With this additional extension the choice between match and try becomes purely stylistic. We might optimise for succinctness, and use try in the case where exceptions are expected (for example, where they’re used for control flow), reserving match for the case where exceptions are truly exceptional.

This feature was worked on between Nov 2013 - Jun 2014.


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