Awesome Match Specs

A community-edited collection of match-spec examples for tracing. Did you find a match-spec that you use often or find tricky? Please add it below so that we can all learn from each other.

The snippets below use XProf-flavoured match-spec fun syntax (either Erlang or Elixir).

Basic Erlang examples

• Only capture ets:lookup calls on table data

ets:lookup(data, _)

• Capture connecting to either TCP port 80 or 443

gen_tcp:connect(_, Port, _, _) when Port =:= 80; Port =:= 443

• Only capture the important field of a possibly big tuple

ets:insert(_, Data) -> message(element(3, Data)).

• And just for the sake of example connecting to either port expressed with multiple clauses. (As the match-spec body is only evaluated for its side-effects or action-function calls and the actual return value is ignored to achieve the default behaviour any dummy term can be put there like ok or true)

gen_tcp:connect(_, 80, _, _) -> true; (_, 443, _, _) -> true end.

Basic Elixir examples

• Capture Registry lookups on MyApp.Registry

Registry.lookup(MyApp.Registry, _)

• Capture dispatching to "topic1" or "topic2"

Registry.dispatch(MyApp.Registry, topic, _) when topic in ["topic1", "topic2"]

• Instead of a possibly long list only capture the length of the list

Enum.fetch(list, index) when is_list(list) -> message([length(list), index])

• Again just for the example dispatching to multiple topics expressed with multiple clauses.

Registry.dispatch(MyApp.Registry, "topic1", _) -> nil; (MyApp.Registry, "topic2", _) -> nil

Caller

Capture lists:foldl/3 calls, but only when it is not called by the sync application. (Sync recompiles and reloads your Erlang code on-the-fly, and calls lists:foldl/3 periodically).

Note that the caller() action-function can only be used in the match-spec body, but there we cannot use pattern matching or if/case statements, so andalso to the rescue.

lists:foldl(_,_,_) -> element(1,caller()) =:= sync_scanner andalso message(false)