[Haskell-cafe] Stupid newbie question
brianh at metamilk.com
Sat Jan 6 09:32:12 EST 2007
Brian Hulley wrote:
> Brian Hulley wrote:
>> Brian Hurt wrote:
>>> nth 0 (x:xs) = Some x
>>> nth i (x:xs) = if i < 0 then Empty else nth (i-1) xs
>>> nth i  = Empty
>> [blows stack on large i]
>> As other people have pointed out, this is due to laziness. I'd write
>> it like:
>> nth 0 (x:_) = Some x
>> nth i (_:xs) = of i < 0 then Empty else (nth $! i-1) xs
> ^^^ -- oops!
>> nth _  = Empty
Actually I see I should have read Jeremy Shaw's answer more carefully before
giving my own since I'd missed the point about the problem being with the
list itself not the decrementing of (i) (which wouldn't be a problem since
it is immediately forced on the next iteration by the comparison of i to 0).
The answer therefore lies in the laziness of the makelist function which
could be solved by:
makelist i = i : (makelist $! i - 1)
Jeremy Shaw wrote:
> pps. I didn't explain why [1..1000000] works, but [1..] fails, because
> I don't know :) It's a bit suprising to me...
I think this might be because the code that produces the list [1..1000000]
would have to be something like:
produce n i = if i < n then i : produce n (i +1) else 
so each element of the list is now forced by the comparison with n, so even
though the end of the list will be a thunk when 1000000 has not yet been
reached, none of the elements are thunks.
Anyway it was certainly not a "stupid" question - thanks for asking it.
Best regards, Brian.
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