CODEWITHSUNDEEP
exceptionerlangpattern-matching
portasynthinca3
portasynthinca3

Reputation: 33

In a sequence of Erlang matches, how do I tell which one of them has failed?

Consider the following snippet:

do_stuff() ->
    % assume each of BoundVarI is bound here
    BoundVar1 = term1(),
    BoundVar2 = term2(),
    % ...
    BoundVarN = termN(),
    ok.

do_stuff_wrapper() ->
    do_stuff().

In the wrapper function (do_stuff_wrapper/0), how exactly do I determine which one of the matches has failed? Just to be clear, I'm not looking for a way to tell that it has just failed, possibly by spawning and monitoring a process, but a way to tell which of the matches has caused to fail. One way I considered would be to just extract the line number from the error tuple, but I feel like that hurts maintainability a lot.

Upvotes: 2

Views: 58

Answers (1)

José M
José M

Reputation: 3509

You could try something along the lines of:

ScopeRef = make_ref(), %% To make sure to catch only errors in this scope
try
  {_, BoundVar1} = {{ScopeRef, term1}, term1()},
  {_, BoundVar2} = {{ScopeRef, term2}, term2()},
  {_, BoundVarN} = {{ScopeRef, termN}, termN()},
  %% ^ Consider turning this into a lists:foreach/recursion
  ok
catch %% First error stops execution
  error:{badmatch, {{ScopeRef, FailedFunName}, _FailedTerm}} -> {error, {nomatch, FailedFunName}}
end.

or, if you want to check every one

BoundTerms = [BoundVar1, BoundVar2, BoundVarN],
AssertFuns = [fun term1/0, fun term2/0, fun termN/0],
FailedTerms = lists:reverse(lists:foldl(fun({BoundVar, AssertFun} = Pair, Acc) ->
  case AssertFun() of
    BoundVar -> Acc;
    _ -> [Pair | Acc]
  end
end, [], lists:zip(BoundTerms, AssertFuns)),
case FailedTerms of
  [] -> ok;
  _ -> exit({assert_failed, FailedTerms})
end.

Depending on the actual problem, I'd choose one or the other (or neither), but these examples show different approaches that you can tweak.

Upvotes: 3

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