[Haskell-cafe] Laziness question

[Nicolas Pouillard]

On Sat, 31 Jul 2010 17:30:54 -0400, Brandon S Allbery KF8NH <allbery at ece.cmu.edu> wrote:
> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA1
> 
> On 7/31/10 16:58 , wren ng thornton wrote:
> > Brandon S Allbery KF8NH wrote:
> >> michael rice wrote:
> >>> Are you saying:
> >>>
> >>> [ head x ]  ->  [ *thunk* ]   and   length [ *thunk* ] ->  1, independent of
> >>> what *thunk* is, even head [], i.e., *thunk* never needs be evaluated?
> >>
> >> Exactly.  (I was being cagey because the first response was cagey, possibly
> >> suspecting a homework question although it seems like an odd time for it.)
> >>
> >> length not only does not look inside of the thunk, it *can't* look inside
> >> it; all it knows is that it has a list, it specifically does *not* know what
> >> that list can hold.  So the only thing it can do is count the number of
> >> "unknown somethings" in the list.
> > 
> > Not entirely true:
> > 
> >     stupidlyStrictLength :: [a] -> Integer
> >     stupidlyStrictLength []     = 0
> >     stupidlyStrictLength (x:xs) = x `seq` 1 + stupidlyStrictLength xs
> 
> Given all the messes seq makes ("hey, go behind the compiler's back and
> touch this arbitrary value of arbitrary type"), I generally consider it to
> be unsafeSeq :)

I would deeply in favor of renaming seq to unsafeSeq, and introduce a
type class to reintroduce seq in a disciplined way.

-- 
Nicolas Pouillard
http://nicolaspouillard.fr