I am currently reading an article[1] where the author uses Haskell to explain her idea. The problem is I don't understand Haskell and am not familiar with Haskell concept like MVar, IVar, etc. (I can get the code compile, and have several years programming experiences.) When testing the code, it throws the error messages "the flag -N2 requires the program to be built with -threaded"; however I can't find thread related flag with ghc command by `ghc --help`. How can I fix this problem? Environment I use: Debian stretch/sid, kernel 4.0.0-2-rt-686-pae, GHC version 7.10.3. The code is exactly the same as described in the section 'Nondeterminism with MVars' of article; and the makefile content is all: ghc -O mvar.hs ghc -o mvar mvar.o run: ./execute # while true; do ./mvar +RTS -N2; done clean: rm mvar Thanks [1]. http://composition.al/blog/2013/09/22/some-example-mvar-ivar-and-lvar-progra...