[Haskell-cafe] Strictness is observable

[Daniel Fischer]

On Friday 01 April 2011 09:00:56, oleg at okmij.org wrote:
> Daniel Fischer wrote:
> > If you have a strict function, you may evaluate its argument eagerly
> > without changing the result^1, while eager evaluation of a non-strict
> > function's argument may produce _|_ where deferred evaluation
> > wouldn't.
> 
> Sadly, that is quite untrue. Strictness is observable, already in
> Haskell98. That distressing result has nothing to do with imprecise
> exceptions, seq, non-termination, lack of resources, or the use of
> unsafe features. Plainly, just by changing the strictness of a
> function one may cause the program to print different results, such as
> "strict" or "non-strict" in the code below.

John Meacham said it's not Haskell98, I can't be bothered to check the H98 
report now, since that's a minor point anyway.

So, mea culpa, I didn't consider catch at all, only thought of non-IO code 
and considered all _|_s equal.
I should have made these restrictions explicit, but I wasn't even 
consciously aware of them.