The correct statement to extract a value from a Random.Generator is Random.step.
As of 0.17 elm-reactor cannot stub out ports that are required by the program it is running.
Elm doesn't have access to the system clock or even the initialisation time of the runtime.
The documentation for Random suggests using system time for creating seeds but this is not available in the standard runtime and even if it was a Github comment suggests it doesn't sufficient spread itself across the bit space to give a good seed.
The recommended way is to use native Javascript's Math.random and pass it via a port.
Math.random() * Math.pow(2,32)
To partial apply a function use the invocation syntax:
(function initial_arg1 initial_arg2 ...)