[Haskell-cafe] Different forms for RandomT?

Zemyla zemyla at gmail.com
Thu Jul 30 17:38:52 UTC 2015

Normally, a monad transformer to provide a random number generator would be
of the form StateT g, where g is a RandomGen. But I've seen some libraries
(like QuickCheck) define their RandomT as:

newtype RandomT g m a = RandomT { runRandomT :: g -> m a }

with their monadic bind operation defined as

(RandomT m) >>= f = RandomT $ \g -> let (ga, gb) = split g in m ga >>= (\a
-> runRandomT (f a) gb)

and return and fail as in ReaderT.

Can someone describe the advantages and disadvantages of doing RandomT this
way? I mean, if your generator has a subpar split operation (and most do),
this will obviously exacerbate any problems with it. Does it give any
comparable advantages?
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