[Haskell-cafe] Lazy patterns

[Scott Turner]

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.