2009/6/20 Stephan Friedrichs :

Deniz Dogan wrote:

...

I (too) often find myself writing code such as this:

if something   then putStrLn "howdy there!"   else if somethingElse           then putStrLn "howdy ho!"           else ...

[...]

So how do I make code like this prettier?

If it's a function, you can use guards:

foo :: ... foo something somethingElse    | something     -> putStrLn "howdy there!"    | somethingElse -> putStrLn "howdy ho!"    | otherwise     -> ...

Good idea, I hadn't thought of that... Currently this is all in the main function, checking the command line arguments. I suppose I could put it in a separate function. Thanks! -- Deniz Dogan

Henning Thielemann wrote:

[...]

http://haskell.org/haskellwiki/Case

Maybe we (i. e. someone with a wiki account ;) ) should add Jeremy's proposal - using let and guards - to the page (under section 2.2, "syntactic suger")? IMHO this is much clearer than "case () of _". foo = let x | 1 > 1 = "uh-oh" | otherwise = "all is well" in x Regards, Stephan -- Früher hieß es ja: Ich denke, also bin ich. Heute weiß man: Es geht auch so. - Dieter Nuhr

At Sat, 20 Jun 2009 20:45:16 +0200, Stephan Friedrichs wrote:

If it's a function, you can use guards:

foo :: ... foo something somethingElse | something -> putStrLn "howdy there!" | somethingElse -> putStrLn "howdy ho!" | otherwise -> ...

You can also artificially introduce places to use gaurds inside a function: foo = let x | 1 > 1 = "uh-oh" | otherwise = "all is well" in x bar = case () of _ | 1 > 1 -> "uh-oh" | otherwise -> "all is well" - jeremy

Antoine Latter wrote:

On Sat, Jun 20, 2009 at 1:05 PM, Deniz Dogan wrote:

...

I (too) often find myself writing code such as this:

if something then putStrLn "howdy there!" else if somethingElse then putStrLn "howdy ho!" else ...

I recall reading some tutorial about how you can use the Maybe monad if your code starts looking like this, but as you can see, that doesn't really apply here. "something" and "somethingElse" are simply booleans and each of them have different actions to take if either of them is True.

So how do I make code like this prettier?

I'm not entirely sure if this is haskell'98, but GHC seems to support this sort of layout:

main = do <some computation>

if something then someOtherComputation else do

<continue at the same indentation> <<<<<

IMHO, this is ugly and counterintuitive; I like having a single "point of exit" (to use an imperative programming term) of a function. Your suggestion is equivalent to someComputation; if something then begin someOtherComputation; exit; end; more; in, say, Pascal. This obscures the fact that "more;" is sometimes/often not executed. (You could argue the same about exceptions, but they are a necessary evil ;-). Regards, -- Jochem Berndsen | jochem@functor.nl GPG: 0xE6FABFAB

Hello Andrew, Sunday, June 21, 2009, 1:52:22 PM, you wrote:

d1x <- doesDirectoryExist d1 if d1x then do f1x <- doesFileExist (d1 f1) if f1x then do d2x <- doesDirectoryExist d2 if d2x then do f2x <- doesFileExist (d2 f2) if f2x then do_stuff d1 d2 f1 f2 else hPutStrLn stderr $ "File " ++ f2 ++ " not found." else hPutStrLn stderr $ "Directory " ++ d2 ++ " not found." else hPutStrLn stderr $ "File " ++ f1 ++ " not found." else hPutStrLn stderr $ "Directory " ++ d1 ++ " not found."

d1x <- doesDirectoryExist d1 if not d1x then hPutStrLn stderr $ "Directory " ++ d1 ++ " not found." else do f1x <- doesFileExist (d1 f1) if not f1x then hPutStrLn stderr $ "File " ++ f1 ++ " not found." else do d2x <- doesDirectoryExist d2 if not d2x then hPutStrLn stderr $ "Directory " ++ d2 ++ " not found." else do f2x <- doesFileExist (d2 f2) if not f2x then hPutStrLn stderr $ "File " ++ f2 ++ " not found." else do do_stuff d1 d2 f1 f2 or, with a little additional combinators: ifM (not ==<< doesDirectoryExist d1) (hPutStrLn stderr $ "Directory " ++ d1 ++ " not found.") $ do ifM (not ==<< doesFileExist (d1 f1)) (hPutStrLn stderr $ "File " ++ f1 ++ " not found.") $ do ifM (not ==<< doesDirectoryExist d2) (hPutStrLn stderr $ "Directory " ++ d2 ++ " not found.") $ do ifM (not ==<< doesFileExist (d2 f2)) (hPutStrLn stderr $ "File " ++ f2 ++ " not found.") $ do do_stuff d1 d2 f1 f2 -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com

Sebastian Fischer wrote:

using Control.Monad.Error:

either (hPutStrLn stderr) return =<< runErrorT $ do d1x <- lift $ doesDirectoryExist d1 unless d1x $ fail "Directory " ++ d1 ++ " not found." f1x <- lift $ doesFileExist (d1 f1) unless f1x $ fail "File " ++ f1 ++ " not found." d2x <- lift $ doesDirectoryExist d2 unless d2x $ fail "Directory " ++ d2 ++ " not found." f2x <- lift $ doesFileExist (d2 f2) unless f2x $ fail "File " ++ f2 ++ " not found." lift $ doStuff d1 d2 f1 f2

So... essentially, run the ErrorT transformer on top of IO to do the error handling? That seems like a much nicer solution - if I can get it to work. (In general, any code involving monad transformers tends to fail with elaborate type checker failures. Typically it takes me hours to figure out why... Maybe somebody has written a tutorial on this stuff?)