[Haskell-beginners] Parse error in pattern
daniel.is.fischer at web.de
Fri Feb 26 12:12:24 EST 2010
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 <flo.morphe at gmail.com>
> > 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
if (length list == 0)
Never. Jamais. Niemals.
if (null list)
length has to traverse the entire list, which may take a long time.
> > then do
> > putStrLn $ "No filename given to the program.\n $
> > ProgramName file.sac"
> > return ()
> I believe the 'do' here is unecessary.
As soon as the unnecessary "return ()" is removed.
> > else
> > sacFile1 <- openBinaryFile fileToOpen ReadMode
> Here you do need a 'do' though, I believe.
Yes. If he binds the name sacFile1 to a value, there must come more
statements after it, so the "do" is required.
But it might also be wrong indentation, if the mail programme fiddled with
> > ghci complains, and tells "Parse error in pattern", indicating the
> > 'if' line number.
Invoke ghci with
$ ghci -ferror-spans file
to see how far GHC thinks the erroneous pattern extends. From that, one can
often deduce better what the problem is.
> > Can you please help ?
> > Thank you very much,
> > Florian
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