keep_zen
Quantum of Elixir or why in rescue it just match the name of the error?
I want define a macro like this.
defmodule Demo do
defmacro maybe(do: block) do
quote do
try do
unquote(block)
rescue
e in [MatchError] -> e.term
# %MatchError{term: term} -> term
e -> reraise e, __STACKTRACE__
end
end
end
end
Why in the rescue it just match the name of the error?
In another words, why can not use the code like %MatchError{term: term} -> term in here?
About 5 years ago there is a question about this.
The different is that I know what is right syntax, but I do not know why it must like that way.
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Qqwy
José Valim happened to talk about it in Reviewing Elixir - Part 4 of the Thinking Elixir podcast.
Briefly summarized, the reason is indeed to make it very hard for people to depend in their rescue clauses on the internals of the exception struct, as this might change over time. (Side note: debugging exceptions being raised because of other exceptions not being recognized properly is very much not fun.)
You can still do it inside the rescue clause if you really really want to, but at that point you’ll have thought really hard about if this really is the approach you want to take and hopefully have reconsidered.
Qqwy
A new mechanism for this was introduced in OTP 24 (c.f. EEP 54). The ‘extra information’ is kept separate from the main value that is being raised, to make sure that existing code did not break.
It was necessary to do it this way in part because the whole error mechanism in Erlang predates maps and was thus solely built on top of atoms and tuples.
msimonborg
One reason may be the internal structure of the exception struct can change over time
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