patch applied (packages/base): Make unsafePerformIO lazy

[Simon Peyton Jones]

Mon May  8 07:25:07 PDT 2006  simonpj at microsoft
  * Make unsafePerformIO lazy
  
  The stricteness analyser used to have a HACK which ensured that NOINLNE things
  were not strictness-analysed.  The reason was unsafePerformIO. Left to itself,
  the strictness analyser would discover this strictness for unsafePerformIO:
  	unsafePerformIO:  C(U(AV))
  But then consider this sub-expression
  	unsafePerformIO (\s -> let r = f x in 
  			       case writeIORef v r s of (# s1, _ #) ->
  			       (# s1, r #)
  The strictness analyser will now find that r is sure to be eval'd,
  and may then hoist it out.  This makes tests/lib/should_run/memo002
  deadlock.
  
  Solving this by making all NOINLINE things have no strictness info is overkill.
  In particular, it's overkill for runST, which is perfectly respectable.
  Consider
  	f x = runST (return x)
  This should be strict in x.
  
  So the new plan is to define unsafePerformIO using the 'lazy' combinator:
  
  	unsafePerformIO (IO m) = lazy (case m realWorld# of (# _, r #) -> r)
  
  Remember, 'lazy' is a wired-in identity-function Id, of type a->a, which is 
  magically NON-STRICT, and is inlined after strictness analysis.  So
  unsafePerformIO will look non-strict, and that's what we want.
  

    M ./GHC/IOBase.lhs -1 +17