In Elm 0.17, I'd like to run a program that relies on random numbers, but I'd like to have a user-specified seed. This is to have reproducible results across multiple user sessions: users who enter the same seed should see the same results.
But I can't figure out how to affect the behavior of built-in functions like:
Random.list 10 (Random.int 0 100)
With a call like the one above, I want to get the same list of 10 random numbers each time I feed in the same seed. But I can't figure out how to feed in a seed at all. I'd appreciate any help!
Generating a random value with Random, using user-specified seed is possible with Random.step
You need to specify a Generator and a Seed, where Generator a is a function to produce random values of a type, using integer Seed
To create a Seed from integer, you need to use Random.initialSeed function, since Seed is not a plain integer, it's a data-structure containing meta-information for the the next steps of the Generator
Generator a -> Seed -> (a, Seed)
Calling Random.step will return a new state (a, Seed), where a is your random value, and Seed is the seed, required for generating the next random value.
I have made a comprehensive example, which shows how to use generator for producing random values: Random value with user-specified seed
The example might be too big for the answer so I will highlight the most important parts:
generator : Int -> Generator (List Int)
generator length =
Random.list length (Random.int 0 100)
The seed might be specified through user input, or you can pass it as flags upon start-up.
Random.step (generator 10) seed