Producers for handling randomness.

Random Numbers

Use functions like uniform and normal to generate endless streams of random numbers of the standard Num types. For instance, you could perform some IO action based on a threshold:

{-# LANGUAGE TypeApplications #-}  -- GHC8 only. Provides the @ syntax.

import qualified Pipes.Prelude as P

perhaps :: Effect IO ()
perhaps = uniform @Float >-> P.filter (> 0.1) >-> lift releaseTheHounds

Random Elements from Containers

We expose the functions finite and endless for randomly Producing elements from a collection.

finite will only Produce until each of its elements have been yielded once. Making a shuffle function then is easy:

import           Data.Vector (Vector)
import qualified Pipes.Prelude as P

shuffle :: Vector a -> IO [a]
shuffle = P.toListM . finite

endless on the other hand will endlessly Produce elements in any order. Repeats will likely appear long before each element has been yielded once. You can limit the number of results with take:

import           Data.Vector (Vector)
import qualified Pipes.Prelude as P

twenty :: Vector a -> Producer a IO ()
twenty v = endless v >-> P.take 20

For the time being, only Vectors (all kinds) are supported.