dimitarvp
Hello,
Is it possible to check if an argument to a guard is a particular structure?
Context: I am trying to have a defguard definition that checks if something is an Ecto query – which means it can either be a module (e.g. atom) or it can be an %Ecto.Query{}. But I cannot seem to find a way to do it via defguard.
defmodule A do
defguard is_query(q) when is_atom(q) or ... # <- what do we put here?
end
Tried with match?(%Ecto.Query{}, q) but that did not compile.
Something I am missing?
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idi527
Just in case, Ecto Queryable is not limited to modules and queries, but can be anything it’s implemented for …
Maybe you could use
Ecto.Queryable.impl_for(term)? It would at least be accurate.dimitarvp
How would you use it in a
defguard?idi527
As for your actual question:
idi527
Why would you need a guard if you have
impl_for? It serves the same function – checking whether something is a queryable, – as far as I understand your goal.dimitarvp
Well, I am trying to find a way to introduce a stricter typing when feeding parameters to my functions.
idi527
Ecto would raise if it receives something that doesn’t implement
Ecto.Queryable… So is it really important to have it fail at the guard to your function instead of at some check within ecto?dimitarvp
I feel it is but having no choice, I’ll accept the
raiseway of doing things by Ecto.dimitarvp
This works on the surface but as you said, it will not capture any other implementations of the
Ecto.Queryable, correct?idi527
Right, it wouldn’t …
dimitarvp
Had to be sure. Thanks for the help!