Hello,
*Proposed feature*
I'd like to propose another improvement on structs. Inspired by @enforce_keys,
I'd like to propose adding @guards which can help to validate the types of
the fields in the struct.
Example usage:
defmodule MyStruct do
@guards [name: :is_binary]
defstruct [:name]
end
which will fail if the given condition is not satisfied:
iex(2)> %MyStruct{name: 5}
** (ArgumentError) The following fields didn't match the guards: struct
MyStruct: [{:name, :is_binary, 5}]
expanding struct: MyStruct.__struct__/1
*Notes*
- As the example shows, the behaviour will be similar to @enforce_keys -
it will be checked only when creating the struct, not when updating
- Using module attribute allows to keep this optional and allows to keep
backwards compatibility
*Possible implementation*
With https://hexdocs.pm/elixir/master/guards.html and Kernel.apply/3, we
can modify existing def __struct__(kv) from Kernel:
def __struct__(kv) do
{map, errors} =
Enum.reduce(kv, {@struct, {[], @enforce_keys}}, fn {key, val}, {map,
{type_errors, key_errors}} ->
guard = @guards[key]
if guard && apply(Kernel, guard, [val]) do
{Map.replace!(map, key, val), {type_errors, List.delete(key_errors,
key)}}
else
{
Map.replace!(map, key, val),
{[{key, guard, val} | type_errors], List.delete(key_errors, key)}
}
end
end)
case errors do
{[], []} -> map
{types, []} ->
raise ArgumentError, "The following fields didn't match the guards: "
<>
"struct #{inspect __MODULE__}: #{inspect types}"
end
end
This, of course, needs style improvements (and validation of required
fields which is currently removed for the sake of clarity), but this is
only a proof of concept to verify that the implementation is possible and
quite easy.
*Why not use @type?*
While it would be cool to be able to verify the types based on typespecs,
it would be harder because I think not all types can be easily validated.
The suggested approach with guards will be feel more familiar because we
can already do this for functions. Adding guard validation for struct
fields feels like reasonable step.
*What do you think?*
I'd be happy to start working on this feature, but I wanted to know what do
you all think about this.
Cheers,
Maciej
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