Re: [Haskell-cafe] Why is this strict in its arguments?

4 Dec 2007

      On Tue, Dec 04, 2007 at 03:35:28PM -0800, Ryan Ingram wrote:
...
On 12/4/07, Stefan O'Rear  wrote:
...
"Is there a reason why 2 + 2 is defined as 4 instead of, for example,
5?"
Wow.  That wasn't really necessary.  4 has a clear meaning (the number after
the number after the number after the number after zero) which is equivalent
to 2 + 2.  I'm not talking about naming issues; you could say that 5 was
that number but then nobody would know what you are talking about.  I am
asking about the history & motivation behind the original definition of
strictness, not arguing for a redefinition.
Oh.  Sorry.
...
...
Strictness is more useful in practice, simpler to define, and easier to
approximate.
Please elaborate; this is exactly why I asked.  In particular, "more useful
in practice" is the thing I am most curious about.
When you see an expression of the form:

f a

you generally want to evaluate a before applying; but if a is _|_, this
will only give the correct result if f a = _|_.  Merely 'guaranteed to
evaluate' misses out on some common cases, for instance ifac:

ifac 0 a = a
ifac n a = ifac (n - 1) (a * n)

ifac is guaranteed to either evaluate a, or go into an infinite loop -
so it can be found strict, and unboxed.  Whereas 'ifac -1 (error "moo")'
is an infinite loop, so using a definition based on evaluation misses
this case.
...
...
What benefit does your notion offer?
Well, one usually says something like "f is strict in its 2nd argument"
which on casual reading tends to make me think that it has something to do
with the argument.  By the actual definition, however, f _ _ = undefined is
strict in all of its arguments; but it's clear from the definition that the
arguments are irrelevant.
Stefan