Consider the following program: module Stack where import System.IO.Unsafe main = print (sim (replicate 1299959 ())) sim [] = True sim (_:xs) = goodStack (sim xs) goodStack x = fromJust (Just x) --no stack overflow badStack x = unsafePerformIO (return x) --stack overflow fromJust (Just x) = x I always thought that unsafePerformIO would do something similar as goodStack. i.e. be stack neutral: $ ghc --make -o stack Stack [1 of 1] Compiling Main ( Stack.hs, Stack.o ) Linking stack ... $ stack +RTS -K0.00001M True But if you exchange goodStack with badStack, the picture changes unfortunately to: $ ghc --make -o stack Stack [1 of 1] Compiling Main ( Stack.hs, Stack.o ) Linking stack ... $ stack +RTS -K9.883647M Stack space overflow: current size 9883644 bytes. Use `+RTS -Ksize' to increase it. $ stack +RTS -K9.883648M True I am using: $ ghc --version The Glorious Glasgow Haskell Compilation System, version 6.8.2 Is this behaviour necessary? Is there any work around, e.g., employing the foreign function interface? Thanks for your time! Bernd