Re: Evaluation order, ghc versus hugs, lazy vs. strict

20 Aug 2002

      On Mon, Aug 19, 2002 at 11:34:48PM +0100, Alastair Reid wrote:
...
...
main = print $ sum [0..1000000]
...
Hugs uses foldl' instead of foldl to define sum:...
Does it really?  That's a violation of the standard: a user's instance
of (+) need not be strict in its left argument.  Consider
...
data Foo = Foo Integer deriving (Eq, Show)
...
instance Num Foo where
  fromInteger = Foo
  a + b = b
...
v1 = undefined + Foo 2
v2 = sum [undefined, Foo 2]
Both v1 and v2 should have the value 'Foo 2'.  However, hugs (December
2001) gives 

Program error: {undefined}

in response to v2.  ghci gets it right.

--Dylan

Re: Evaluation order, ghc versus hugs, lazy vs. strict

Dylan Thurston