Core Language


These consist of a path into the output object, an equals sign, and an expression of what to put in that path.

my.output.field = some.input.field

The clear benefit of this is it handles object hierarchies pretty well. For example:

  • If my or output don’t exist then the nested structure is generated
  • You wont get null-pointer exceptions if the right-hand side is missing. The field is simply missing from output unless you assign it to some other value or set a default

Temporary Variables

These are used to store expressions which are used while processing the item, they use the keyword let to find a variable which is not insert into the final output.

let DATEFORMAT = "yyyy-MM-dd"


Expressions are sections of code which can evaluate, these are covered in depth in the Expressions section.


Predicates are sections of code which can evaluate to a true or false, and are used for control flow. These are covered in depth in the Predicates section.


These are used to separate sections of a mapping, if there are none, the entire document is put inside a [main] block automatically, if you use blocks you must designate your own main block.

twitter            = root.apply("tweet")

id                 = id_str

You can apply blocks with the apply() function as shown in the example.


Inside the body of a template:

  • The left-hand side is points to a new object
  • The right-hand side is points to the object the template is applied to

The above makes templates very effective for re-using key bits of functionality.

So in the earlier example:

  • main iterates the data being submitted and returns the interaction
  • tweet iterates either the top level objects or their retweeted_status and returns the retweet and retweeted objects

You can use the root and this keyword if you want to be explicit about scope.