Search code examples
erlangetsparse-transform

erlang: function called with real 'fun' should be transformed with parse_transform?

I'm looking at the O'Reilly Erlang Programming book and there's an example that is run in the erlang shell that looks like this:

17> MS = ets:fun2ms(fun({Name,Country,Job}) when Job /= cook ->
                   [Country,Name] end).
[  ....an erlang match expression is returned....  ]
18> ets:select(countries, MS).
[[ireland,sean],[ireland,chris]]

However, when I do something similar in my code (not in the shell):

Fun = fun({Type,_,_,ObjectId,PlayerId}) when Type==player_atom, PlayerId==2 -> ObjectId end,
MatchFun = ets:fun2ms(Fun),
PlayerObjectId = ets:select(?roster_table, MatchFun),

I get FUBAR:

exit:{badarg,{ets,fun2ms,[function,called,with,real,'fun',should,be,transformed,with,parse_transform,'or',called,with,a,'fun',generated,in,the,shell]}}

(As an aside, I wonder why the error isn't 'function called with....' Probably so io:format("~p", TheErrorMessage) will line wrap?)

Anyway, I have abandoned select in favor of ets:foldl, since the latter works and - through exceptions in the fun - allows me to terminate the traversal when the first item is found. But, I'm still curious...

...wha? (I did some reading on parse_transform, and I'm new enough to erlang that I'm missing the connection.)

Solution

  • The badarg exception is symptom of a built-in function (or a pseudo function, as in this case) called with a wrong parameter. In this case, the ets:fun2ms/1 function.

    Reading from the official documentation:

    fun2ms(LiteralFun) -> MatchSpec

    Pseudo function that by means of a parse_transform translates LiteralFun typed as parameter in the function call to a match_spec. With "literal" is meant that the fun needs to textually be written as the parameter of the function, it cannot be held in a variable which in turn is passed to the function).