Re: [Haskell-cafe] Re: Web server continued

31 Dec 2007

      On Dec 31, 2007, at 6:50 AM, Cristian Baboi wrote:
...
On Mon, 31 Dec 2007 14:36:02 +0200, Joost Behrends 
...
wrote:
...
I forgot 2 things:
...
The distinction between '=' and '==' is much like in C, although  
mixing
them up is not so dangerous like in C. ':=' and '=' like in Wirth
languages would be nicer.
...
Strangely nobody reacted on this. That a=a+1 is an infinite  
recursion here
What is more strange is that a = a + 1 and a = 1 + a are somehow  
distinct.
The second give a stack overflow almost instanly, but the first don't.
To explain this, let's look at the expansions. The first one looks  
like this:

      a + 1
      a + 1 + 1
      a + 1 + 1 + 1
      a + 1 + 1 + 1 + 1
      ...
      a + 1 + ... + 1

The second one looks like this:

      1 + a
      1 + 1 + a
      1 + 1 + 1 + a
      1 + 1 + 1 + 1 + a
      ...
      1 + ... + 1 + a

I suspect that GHC is evaluating the second one strictly, as though it  
was this instead:

      a = foldl1' (+) (repeat 1) + a

This way, the stack never gets any larger. The first one, however, has  
runtime behavior comparable to this:

      a = foldl' (+) a (repeat 1)

The first thing this version has to do is evaluate a, which starts  
infinite recursion. The second one doesn't suffer from this because it  
has real values to manipulate strictly instead of just substitution  
rules and thunks.

I'm speculating here, so somebody please correct me if I'm wrong.

- Jake

Re: [Haskell-cafe] Re: Web server continued

Jake McArthur