List FromConfig

This FromConfig instance provides support for parsing haskell's lists.

Parsing a configuration as a list will use the keys to sort the values lexicographically and it will parse each value using the FromComfig instance associated to the type of the elements the list contains.

allowed_ips.01=101.125.246.105
allowed_ips.02=7.173.68.54

fetching a list of String from the following config using the key "allowed_ips" would return ["101.125.246.105", "7.173.68.54"]

allowed_ips.ludat=7.173.68.54
allowed_ips.juan=101.125.246.105

fetching a list of String from the following config using the key "allowed_ips" would return ["101.125.246.105", "7.173.68.54"].

Special Keys

keys

If you don't want to use the default lexicographic order, you can add a keys key to the config with a comma separated list consisting of the other keys in the config. The order in the parsed list will be the same as the order in that list.

allowed_emails.ludat[email protected]
allowed_emails.juan[email protected]
allowed_emails.keys=ludat,juan

fetching a list of String from that config using the key "allowed_emails" would return ["[email protected]", "[email protected]"].

The value of keys does not need to have all other keys in the configuration. If some key is missing then it won't appear in the parsed list.

allowed_emails.ludat[email protected]
allowed_emails.juan[email protected]
allowed_emails.keys=ludat

fetching a list of String from the following config using the key "allowed_emails" would return ["[email protected]"]

Specifying keys that are not present in the configuration will result in an error.

allowed_emails.ludat[email protected]
allowed_emails.juan[email protected]
allowed_emails.keys[email protected]

fetching a list of String from that config using the key "allowed_emails" would raise an error saying that the key facu was not found.

Finally, you can reference the defaults in keys by index like defaults.0, defaults.1, etc.

allowed_emails.ludat[email protected]
allowed_emails.juan[email protected]
allowed_emails.keys=ludat,juan,defaults.0

if we had a default list of emails configured as ["[email protected]"], we could fetch a list of String from the config using the key "allowed_emails" and get ["[email protected]", "[email protected]", "[email protected]"]

prototype

Sometimes you might have more complex objects in your keys that need to have their own subkeys. For example, if we defined an Email type like this one:

data Email = Email { username :: String, domain :: String } deriving (Generic, Show)

instance FromConfig Email

Then the config that we had in the last examples won't work, we'll need to rewrite this:

allowed_emails.ludat[email protected]
allowed_emails.juan[email protected]

into:

allowed_emails.ludat.username=ludat
allowed_emails.ludat.domain=aol.com
allowed_emails.juan.username=juan
allowed_emails.juan.domain=aol.com

And when we fetch a list of Email using the key "allowed_emails" we will get [Email "juan" "aol.com", Email "ludat" "aol.com"].

Also, let's say that in this app most allowed emails share the same domain. In order to avoid repeating it every time, we can use the key prototype like this:

allowed_emails.ludat.username=ludat
allowed_emails.juan.username=juan
allowed_emails.prototype.domain=aol.com

That config will yield the same list of emails than the one before. The prototype's domain value is used for all keys that do not have a domain set explicitly.

If there's a key that already has a domain set, that one will be used:

allowed_emails.ludat.username=ludat
allowed_emails.juan.username=juan
allowed_emails.facu.username=facu
allowed_emails.facu.domain=yahoo.com
allowed_emails.prototype.domain=aol.com

that config results in [Email "facu" "yahoo.com", Email "juan" "aol.com", Email "ludat" "aol.com"].

It's not the most usual thing to do, but if a prototype fully defines a value, it will be used for any unknown key:

allowed_emails.ludat.username=ludat
allowed_emails.juan.username=juan
allowed_emails.prototype.username=example
allowed_emails.prototype.domain=aol.com
allowed_emails.keys=facu,juan,ludat,joaco

that config results in [Email "example" "yahoo.com", Email "juan" "aol.com", Email "ludat" "aol.com", Email "example" "yahoo.com"].