translation should allow to be ignored

[Licenser]

Basically it is possible to have a mapping not take effect when they are not present, right now there isn't a way to have this behaviour for translations.

A simple way to implement this would to change the return value of translation functions to {ok, Value} or undefined in which case undefined would mean the translation has no effect.

[joedevivo]

@Licenser could you include the real-world example you ran in to?

[Licenser]

• The mDNS library I use auto detects the NIC to use unless a IP to use is specified in the app.config.
• The main app will use either a autodetected or a specified IP/Port combination.

In the schema I tried to assing the mdns ip based on weather the ip/port is set, this needed a translation, which should not set anything in the app.config when ip/port is not set.

[joedevivo]

@Licenser Would having the translation return 'undefined' when the IP/Port combination solve the problem?

[Licenser]

Yap, it would turn into {ok, undefined} with the get_env which is different form undefined aside of adding a third fork in the code path it'd break the claim that we can use get_env the same way as before ;)

[joedevivo]

@Licenser what if you used get_env/3 instead of get_env/2?

[Licenser]

Not available in erlang R15 :(, that said in R16 I could put in the code to default to {ok, undefined} but that still would be kind of a horrible hackish solution.

[joedevivo]

So it's no longer 'undefined' it's "defined as undefined"

[Licenser]

yea but get_env treats those two situations differently:

• if it is undefined get_env(something) returns undefined
• if it is defined as undefined then get_env(something) returns {ok, undefined}

[joedevivo]

I'll take this scenario under advisement, but for now, your best bet is to add the third clause for reading this value:

{ok, undefined} -> blah;
undefined -> blah;
{ok, Value} -> blah

[Licenser]

To add to this I just thought of another issue, if someone were to use get_env/3 the default would not be picked when the value is set to undefined instead of not being defined at all breaking that behavior and being quite counter intuitive.

[joedevivo]

get_env/3 has it's own issues, use at your own risk ;)

[seancribbs]

@Licenser We now have an API function cuttlefish:unset() which can be used to skip emitting a value to the generated configuration. Closing this issue.