Re: [Haskell-beginners] Parse error in pattern

On Fri, Feb 26, 2010 at 5:12 PM, Daniel Fischer wrote:

Am Freitag 26 Februar 2010 17:37:30 schrieb Magnus Therning:

...

It doesn't look like a complete piece of code so these comments aren't backed up by running it through GHCi or anything.

On Fri, Feb 26, 2010 at 16:29, Florian Duret wrote:

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Hello,

I try to set up a verification on the number of arguments given to my program, but keep on getting "Parse error in pattern" Here is what my code looks like: main :: IO() main = do -- On commence par ouvrir le fichier SAC en mode binaire argsList <- getArgs if (length (argsList) == 0)

It's most likely harmless for argument lists (although there are other cases), but

Don't Use

if (length list == 0)

Never. Jamais. Niemals.

Use

if (null list)

length has to traverse the entire list, which may take a long time.

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. Bob