Hello forum!
I’m trying to retrieve an updated conn, returned in the first line of a “with” statement, when second line failed. But I’m getting some error that says new_conn variable doesn’t exist. In the “do” block it is fine though.
Let me show you what my code looks like:
def some_action(conn, params) do
with {:ok1, some_result, new_conn} <- some_func(conn, params),
{:ok2, another_result} <- another_func(some_result)
do
new_conn |> redirect(to: some_path(conn, :index)) # new_conn is accessible here
else
{:error1, message} -> render(conn, "some_error_template.html")
# new_conn is not accessible here
{:error2, message} -> render(new_conn, "other_error_template.html")
end
end
Please how do you think I could get new_conn in the “else” block?
I have an idea but I’m wondering if it is not bad to use a function such as Tuple.append/2 to ensure the new_conn is present in the matching tuple…
Ok that way I keep the current code, simple and short. I will just writte one more clause for the function that returns {:error2, message} for it takes one more parameter (conn) and just returns it with its own data.
Thank you ^^
I fell into a similar issue and I don’t want to create new functions that return wanted variables in function.
Let’s take this simple snippet:
iex(1)> with a <- 1,
...(1)> 2 <- 3 do
...(1)> :ok
...(1)> else error ->
...(1)> {:error, binding()}
...(1)> end
{:error, [error: 3]}
You can see that the"a" variable is not bound to the else block context, there’s only the “error” one.
If we go with the solution given in the previous comments, we have to introduce a function that returns “a” but you will easily agree that doing so for a such a simple piece of code is stupid:
iex(5)> f = fn(x) -> {3, x} end
#Function<6.50752066/1 in :erl_eval.expr/5>
iex(6)> with a <- 1,
...(6)> 2 <- f.(a) do
...(6)> :ok
...(6)> else error ->
...(6)> {:error, binding()}
...(6)> end
{:error, [error: {3, 1}, f: #Function<6.50752066/1 in :erl_eval.expr/5>]}
I found another solution by using process dictionary:
iex(1)> with a <- 1,
...(1)> _ <- Process.put(:a, a),
...(1)> 2 <- 3 do
...(1)> :ok
...(1)> else error ->
...(1)> {:error, error, Process.get(:a)}
...(1)> end
{:error, 3, 1}
I don’t care about errors.
As in the original question, I would like to get all variables assigned in the first with clauses (before something went wrong) from the else block.
based on this thread, I ended by creating a kind of accumulator that gathers all results got during the multiple clauses
this more generic solution looks like:
with {{:ok, res1}, acc} <- {fun1(), []},
{{:ok, res2}, acc} <- {fun2(), acc ++ [res1]},
...
{{:ok, resN}, acc} <- {funN(), acc ++ [resN-1]} do
# do some stuff...
else
{error, acc} ->
# error contains the result of funP
# acc contains the results of fun1 to funP-1
# do some other stuff...
end
i too am a bit surprized the bindings in the do arent accessable in the else, given the my interpretation of the purpose of the with.
so … is @ntalfer solution still the “best” one? the only other way ive found after a bit of research is to implement some sort of railway oriented version, which is certainly non-trivial just to get access to the bindings.
