[Haskell-cafe] Laziness question

[michael rice]

Subtle stuff.

Thanks, everyone, for your patience. You've been VERY helpful. Great list!

Michael

--- On Sat, 7/31/10, Brandon S Allbery KF8NH <allbery at ece.cmu.edu> wrote:

From: Brandon S Allbery KF8NH <allbery at ece.cmu.edu>
Subject: Re: [Haskell-cafe] Laziness question
To: haskell-cafe at haskell.org
Date: Saturday, July 31, 2010, 2:29 PM

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On 7/31/10 14:24 , 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.

- -- 
brandon s. allbery     [linux,solaris,freebsd,perl]      allbery at kf8nh.com
system administrator  [openafs,heimdal,too many hats]  allbery at ece.cmu.edu
electrical and computer engineering, carnegie mellon university      KF8NH
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