<div class="gmail_quote">On Fri, Feb 26, 2010 at 5:12 PM, Daniel Fischer <span dir="ltr"><<a href="mailto:daniel.is.fischer@web.de">daniel.is.fischer@web.de</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex;">
Am Freitag 26 Februar 2010 17:37:30 schrieb Magnus Therning:<br>
<div class="im">> It doesn't look like a complete piece of code so these comments aren't<br>
> backed up by running it through GHCi or anything.<br>
><br>
> On Fri, Feb 26, 2010 at 16:29, Florian Duret <<a href="mailto:flo.morphe@gmail.com">flo.morphe@gmail.com</a>><br>
wrote:<br>
> > Hello,<br>
> ><br>
> ><br>
> > I try to set up a verification on the number of arguments given to my<br>
> > program, but keep on getting "Parse error in pattern"<br>
> > Here is what my code looks like:<br>
> > main :: IO()<br>
> > main = do<br>
> > -- On commence par ouvrir le fichier SAC en mode binaire<br>
> > argsList <- getArgs<br>
> > if (length (argsList) == 0)<br>
<br>
</div>It's most likely harmless for argument lists (although there are other<br>
cases), but<br>
<br>
Don't Use<br>
<br>
if (length list == 0)<br>
<br>
Never. Jamais. Niemals.<br>
<br>
Use<br>
<br>
if (null list)<br>
<br>
length has to traverse the entire list, which may take a long time.</blockquote><div><br></div><div>While the advice here is sound 90% of the time, it's evidence of falling into the trap of underestimating lazyness. If 0 and length list are both of type Nat, the first will be exactly as fast as the second.</div>
<div><br></div><div>Bob </div></div>