[Haskell-cafe] do you have to use fix with forkio?
lrpalmer at gmail.com
Fri Mar 6 16:33:00 EST 2009
On Fri, Mar 6, 2009 at 1:48 AM, Daryoush Mehrtash <dmehrtash at gmail.com>wrote:
> Question: Do I need to worry about space leak if I am using the fix to
> instead of the "let"?
If you need to worry about a space leak with fix, you need to worry about it
The reason arrows can tie the loop tighter is more about the nature of
recursion in streams; an arrow "sees" that prior values of a signal are not
used, whereas value recursion is much less restricted. If, for example, the
arrow were a kleisli arrow over the list monad, this would not be possible.
With the definition fix f = let x = f x in x, you should not see any
performance difference, other than the standard HOF penalty if there is not
enough inlining... but that should not be asymptotic anyway.
> 2009/3/5 Luke Palmer <lrpalmer at gmail.com>
>> On Thu, Mar 5, 2009 at 6:27 PM, Donn Cave <donn at avvanta.com> wrote:
>>> Quoth Jonathan Cast <jonathanccast at fastmail.fm>:
>>> > You can certainly use let:
>>> > reader <- forkIO $ let loop = do
>>> > (nr', line) <- readChan chan'
>>> > when (nr /= nr') $ hPutStrLn hdl line
>>> > loop
>>> > in loop
>>> > But the version with fix is clearer (at least to people who have fix in
>>> > their vocabulary) and arguably better style.
>>> Would you mind presenting the better style argument? To me, the
>>> above could not be clearer, so it seems like the version with fix
>>> could be only as clear, at best.
>> I like using fix when it's simple rather than let, because it tells me the
>> purpose of the binding. eg., when I see
>> let foo = ...
>> Where ... is fairly long, I'm not sure what the purpose of foo is, or what
>> its role is in the final computation. It may not be used at all, or passed
>> to some modifier function, or I don't know what. Whereas with:
>> fix $ \foo -> ...
>> I know that whatever ... is, it is what is returne, and the purpose of foo
>> is to use that return value in the expression itself.
>> I know that it's a simple matter of scanning to the corresponding "in",
>> but let can be used for a lot of things, where as fix $ \foo is basically
>> only for simple knot-tying. Now, that doesn't say anything about the use of
>> fix without an argument (passed to an HOF) or with a tuple as an argument or
>> many other cases, which my brain has not chunked nearly as effectively. I
>> think fix is best with a single, named argument.
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