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

[Ryan Ingram]

On 12/4/07, Stefan O'Rear <stefanor at cox.net> wrote:

> 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.


By this line:
you generally want to evaluate a before applying; but if a is _|_, this
will only give the correct result if f a = _|_

I assume you mean that it's generally more efficient to do things that way,
because the calling function may have more information about "a" or how it
is calculated, so you may be able to optimize better by doing eager
evaluation whenever it is correct.

Your ifac example makes perfect sense, thanks.

  -- ryan
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://www.haskell.org/pipermail/haskell-cafe/attachments/20071204/350e910a/attachment.htm