PropCheck.BasicTypes.function

You're seeing just the function function, go back to PropCheck.BasicTypes module for more information.
Link to this function

function(arg_types, return_type)

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Specs

function([raw_type()] | arity(), raw_type()) :: type()

All pure functions that map instances of arg_types to instances of ret_type.

The syntax function(arity, ret_type) is also acceptable.