Re: [Haskell-beginners] Parse error in pattern

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.

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       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.

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      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.

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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.

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Can you please help ? Thank you very much, Florian