How to understand such a `case` ?

older
"Rebox Package" or "To Hackage or...

zaxis

8 Dec 2009 8 Dec '09

7:59 p.m.

findHelper (x:xs) = do -- not lazy, but that's not really important here filex <- fileExists (file x) filex' <- fileExists (file' x) case () of _ | filex -> return $ Just $ file x | filex' -> return $ Just $ file' x | otherwise -> findHelper xs file x = foldl1 joinFileName (x ++ [helper]) file' x = (file x) ++ (getConfig "exe_ext") Sincerely! ----- fac n = foldr (*) 1 [1..n] -- View this message in context: http://old.nabble.com/How-to-understand-such-a-%60case%60---tp26703526p26703... Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

Show replies by date

John Millikin

8 Dec 8 Dec

8:11 p.m.

Sorry; forgot to CC the list That case is equivalent to: if filex then return $ Just $ file x else if filex' then return $ Just $ file' x else findHelper xs The specific syntax being used is called a "pattern guard": http://www.haskell.org/haskellwiki/Pattern_guard On Tue, Dec 8, 2009 at 16:59, zaxis wrote:

...

findHelper (x:xs) = do -- not lazy, but that's not really important here filex <- fileExists (file x) filex' <- fileExists (file' x) case () of _ | filex -> return $ Just $ file x | filex' -> return $ Just $ file' x | otherwise -> findHelper xs file x = foldl1 joinFileName (x ++ [helper]) file' x = (file x) ++ (getConfig "exe_ext")

Sincerely!

----- fac n = foldr (*) 1 [1..n] -- View this message in context: http://old.nabble.com/How-to-understand-such-a-%60case%60---tp26703526p26703... Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

zaxis

8:26 p.m.

thanks for your quick answer ! Then what's the advantage using such a `case` not using `if` statement given by you ? For me, the `if` statement is more clear . jmillikin wrote:

...

Sorry; forgot to CC the list

That case is equivalent to:

if filex then return $ Just $ file x else if filex' then return $ Just $ file' x else findHelper xs

The specific syntax being used is called a "pattern guard": http://www.haskell.org/haskellwiki/Pattern_guard

On Tue, Dec 8, 2009 at 16:59, zaxis wrote:

...
findHelper (x:xs) = do -- not lazy, but that's not really important here filex <- fileExists (file x) filex' <- fileExists (file' x) case () of _ | filex -> return $ Just $ file x | filex' -> return $ Just $ file' x | otherwise -> findHelper xs file x = foldl1 joinFileName (x ++ [helper]) file' x = (file x) ++ (getConfig "exe_ext")

Sincerely!

----- fac n = foldr (*) 1 [1..n] -- View this message in context: http://old.nabble.com/How-to-understand-such-a-%60case%60---tp26703526p26703... Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

----- fac n = foldr (*) 1 [1..n] -- View this message in context: http://old.nabble.com/How-to-understand-such-a-%60case%60---tp26703526p26703... Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

Daniel Fischer

9:07 p.m.

Am Mittwoch 09 Dezember 2009 02:26:11 schrieb zaxis:

...

thanks for your quick answer ! Then what's the advantage using such a `case` not using `if` statement given by you ? For me, the `if` statement is more clear .

Once you have a lot of possibilities to check, if-then-else cascades become rather unreadable. However, it might then be advisable to rethink your code, perhaps introduce a choice combinator.

Daniel Fischer

8:35 p.m.

Am Mittwoch 09 Dezember 2009 01:59:21 schrieb zaxis:

...

findHelper (x:xs) = do -- not lazy, but that's not really important here filex <- fileExists (file x) filex' <- fileExists (file' x) case () of _

| filex -> return $ Just $ file x | filex' -> return $ Just $ file' x | otherwise -> findHelper xs

Such a 'case' is a typographically more pleasant way to test multiple alternatives than a nested if-then-else chain. With an if-then-else chain, the code would wander to the right and be less easily followed. Using a case () of _ | condition1 -> thing1 | condition2 -> thing2 | condition3 -> thing3 ... you can nicely align all possibilities. Since the 'case' here is extraneous to the code logic and serves only aesthetic ends, such a practice may be frowned upon.

...

file x = foldl1 joinFileName (x ++ [helper]) file' x = (file x) ++ (getConfig "exe_ext")

Sincerely!

5642

Age (days ago)

5642

Last active (days ago)

List overview

Download

4 comments

3 participants

participants (3)

Daniel Fischer
John Millikin
zaxis