Haskell token bucket rate limiter

import Data.IORef (IORef, atomicModifyIORef', newIORef)
import GHC.Clock (getMonotonicTime)

data Bucket = Bucket { available :: Double, lastRefill :: Double }

newBucket :: Double -> IO (IORef Bucket)
newBucket capacity = do
  now <- getMonotonicTime
  newIORef (Bucket capacity now)

tryAcquire :: Double -> Double -> Double -> IORef Bucket -> IO Bool
tryAcquire capacity rate cost ref = do
  now <- getMonotonicTime
  atomicModifyIORef' ref (refill now)
  where
    refill now b =
      let refilled = min capacity (available b + (now - lastRefill b) * rate)
      in if refilled >= cost
           then (Bucket (refilled - cost) now, True)
           else (Bucket refilled now, False)