make selecting with tuples easier

[breunigs]

Currently, when selecting with a tuple in the guards, one gets a "badarg" error. Generally speaking, the error message is not very helpful because it 1) doesn't specify where 2) dumps Erlang match specification which is complex to read.

This of course stems from Erlang trying to interpret the tuple as a match specification, instead of as a literal. However, the specification states that {:const, <original value here>} can be used to mark literals. I.e., a guard in memento syntax like this fails:

{:==, :year, {1993, 1994}}

whereas this succeeds (query works, but is non-sensical with the test suite schema):

{:==, :year, {:const, {1993, 1994}}}

I did a superficial test with memento's test suite, and updating the spec translator at

memento/lib/memento/query/spec.ex

Lines 58 to 60 in f06bc68

	defp translate(_map, term) do
	term
	end

to emit {:const, <literal>} instead of simply <literal> makes the first example "just work" without breaking any of the other test cases.

I have no experience with Erlang's match spec, and this almost seems too easy to not have any side effects I'm simply not aware of. And of course, changing it now in Memento is a breaking change.

However, at least mentioning this trick in the select/3 documentation would be appreciated, to avoid having to read up on the match spec docs for slightly more complicated than normal cases.

[fabio-t]

I just hit this kind of issue - thank you for this @breunigs!

A fix in Memento would be really appreciated.