[Haskell-cafe] Laziness question

[Brandon S Allbery KF8NH]

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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 :)

- -- 
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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