Re: [Haskell-beginners] Parse error in pattern

26 Feb 2010

      It seems that both your suggestions have worked ! Thank you very much. But I
still can't figure out what went wrong.

My initial goal was to keep the minimum inside the if ... then ... else
statement. Basically, if the list is empty, then stop. If not, then assign
the argument to sacFile1, and go on with the rest.

Here is what it looks like now:
module Main () where

import System.IO
import System.Environment(getArgs)
import Data.Char(intToDigit)

import SAC_Type
import SAC_IO

main :: IO()
main = do
     -- On commence par ouvrir le fichier SAC en mode binaire
     argsList <- getArgs
     if (null argsList)
        then
              putStrLn $ "No filename given to the program.\n $ ProgramName
file.sac"
        else do
           sacFile1 <- openBinaryFile (head argsList) ReadMode

           position <- hTell sacFile1
           putStrLn $ "Position 1: " ++ [intToDigit( fromInteger (position)
)]

           hSeek sacFile1 AbsoluteSeek 440
           position2 <- hTell sacFile1
           putStrLn $ "Position 2: " ++ [intToDigit( fromInteger (position2)
)]

           -- A la fin, il faut evidemment le fermer
           hClose sacFile1

Thank you, Danke, 谢谢, merci, etc...

2010/2/26 Daniel Fischer 
...
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:
...
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.
...
...
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
that.
...
...
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