!= package:aeson

Helper for use in combination with .:? to provide default values for optional JSON object fields. This combinator is most useful if the key and value can be absent from an object without affecting its validity and we know a default value to assign in that case. If the key and value are mandatory, use .: instead. Example usage:

v1 <- o .:? "opt_field_with_dfl" .!= "default_val"
v2 <- o .:  "mandatory_field"
v3 <- o .:? "opt_field2"

Retrieve the value associated with the given key of an Object. If the key is not present and the omittedField is Just x for some x, the result will be that x. This differs from .:?= by attempting to parse Null the same as any other JSON value, instead of using omittedField when it's Just.