Pattern matching is the answer. 
Instead of:
if user && user.address && user.address.city do
...
else
...
end
do:
case user do
%{address: %{city: city}} when is_binary(city) -> ...
_ -> ...
end
josevalim
Creator of Elixir
20
sudostack
Thanks @josevalim for sharing the idea for the below as well. This works better in our Slime partials:
@doc """
Save navigation for structs:
struct_get_in user, [:address, :city]
"""
def struct_get_in(data, key) when is_atom(key) do
struct_get_in(data, [key])
end
def struct_get_in(data, keys) when is_list(keys) do
{first, last} = Enum.split(keys, -1)
first = Enum.map(first, &Access.key(&1, %{}))
last = Enum.map(last, &Access.key(&1, nil))
get_in data, first ++ last
end
1
ku1ik
For cases where we may get either a struct or nil, this is another way of handling it, without case or extra function heads:
struct_or_nil
|> Kernel.||(%{})
|> Map.get(:a_key) # works for both struct and bare map
1
LostKobrakai
ku1ik
LostKobrakai
josevalim
Creator of Elixir
tomekowal
@josevalim I don’t believe that is true.
If you have something like this:
o = %OuterStruct{
inner: %InnerStruct{
field: 1
}
}
get_in o, [Access.key(:inner), Access.key(:field)] #returns 1
o = %OuterStruct{
inner: nil
}
get_in o, [Access.key(:inner), Access.key(:field)]
# returns
** (BadMapError) expected a map, got: nil
(elixir 1.12.3) lib/map.ex:464: Map.get(nil, :pairing, :a)
(elixir 1.12.3) lib/access.ex:459: anonymous fn/5 in Access.key/2
This happens because Access.key returns default only for a missing key. The :inner key is not missing, it is just set to nil so it is passed to next accessor.
1
josevalim
Creator of Elixir
I get a different behaviour:
iex(2)> defmodule S do
...(2)> defstruct [:inner]
...(2)> end
iex(3)> get_in %S{inner: nil}, [Access.key(:inner), Access.key(:field)]
nil
iex(4)> get_in %S{inner: nil}, [Access.key(:unknown), Access.key(:field)]
nil
But i am on v1.13-rc.0. Maybe we fixed something?
2
