Haskell does not garbage collect the head of a list?

Question

Consider the following program:

module Main where

import Control.Monad.List

main = runListT $ do
  x <- ListT $ return $ [0..1000000000]
  lift $ print x

Ideally, we would want the list to be garbage collected as we consume it, so that this program only uses constant memory. But when I compile and run it with

ghc Main.hs -O2 -o Main

I see that it keep using more and more memory. How do I convince Haskell to GC the consumed elements of the list?

Answer 1

The ListT in transformers doesn't stream or run in constant space. The ListT in pipes does!

import Control.Monad (mzero)
import Pipes

main = runListT (do
    x <- Select (each [0..1000000000])
    lift (print x)
    mzero )

I also just uploaded pipes-4.1.4 today, which relaxes runListT to not require the mzero at the end, so then it would just be:

-- Requires `pipes-4.1.4`
import Pipes

main = runListT (do
    x <- Select (each [0..1000000000])
    lift (print x) )