On 2015-06-10 10:00, Jon Schneider wrote:

I am trying to understand the lazy patterns as explained at

https://www.haskell.org/tutorial/patterns.html

If I type in the example without putting a ~ round the pattern sure enough I do not get an answer as the text says. What I completely fail to understand is why there is no CPU usage, increasing memory usage, stack overflow or other runtime error before I use ^C to stop it. What is going on under the hood ?

If you compile the program to binary before executing, the output is progname: <<loop>> See http://stackoverflow.com/questions/21505192/haskell-program-outputs-loop for an explanation of what the ghc runtime has detected. In your case you're using ghci or runhaskell, which has "optimized" the CPU usage rather than raise an exception.