The reason :ets.fun2ms/1 works like this is because it is basically a macro. To use this in Erlang you need to include a -include_lib("stdlib/include/ms_transform.hrl"). in your Erlang file. (it’s in the docs) This runs a parse transform which at compile-time transforms the fun into the match spec. If this transformation is not done at compile time then the :ets.fun2ms/1 function is called at run-time and it gives that rather clumsy error message.
This include also the :dbg.fun2ms/1 “macro” which is used to build match specs for tracing. They are very similar to the ones for tables but not quite the same.
Parse transforms are like macros on steroids. The parse transform’s module/2 function is called with the whole Erlang module AST as an argument with which it can do whatever it likes. It just has to return a new complete module AST. Quite fun once you get the hand of it.
