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. -- Haskell98! -- Strictness is observable -- With no seq, no unsafe operations import Control.Exception -- fs and fns are both essentially (const True) functions, -- but differ in strictness fs,fns :: Bool -> Bool -- non-strict fns x = True -- strict fs True = True fs x = True handler :: SomeException -> IO () handler _ = print "strict" test f = handle handler $ if f (error "Bang!") then print "non-strict" else return () main_s = test fs -- prints "strict" main_ns = test fns -- prints "non-strict"