Oke, So this cannot be done with guards ? Roelof Max Voit schreef op 9-11-2014 18:04:

Hi Roelof,

the problem is, you cannot pattern-match within guards. Guards take predicates.

On your example: "last3 a" pattern-matches the first argument of last3 into a. You may now use a in all further statements. If, however, you want to do pattern matching, it would look like this:

last3::[a]-> Maybe a; last3 [] = Nothing last3 ([a]) = Just a last3 (_:xs) = last3 xs

Notice that the last case will never be executed, as the matching is complete with the first two case.

Max

Am Sun, 09 Nov 2014 17:38:26 +0100 schrieb Roelof Wobben :

...

Hello,

Im trying to find several answers to problem1 of the 99 haskell problems. Find the last item in a list.

Now im trying to find a guarded solution. So far I have this:

last3::[a]-> Maybe a; last3 a | [] = Nothing | ([a]) = Just a | (_:xs) = last3 xs

But I see this error messages :

src/Main.hs@10:8-10:10Not in scope: xs

How to solve this ?

Roelof

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On Sun, Nov 9, 2014, at 06:10 PM, Roelof Wobben wrote:

So this cannot be done with guards ?

Here is one way to do this "with guards": last3 :: [a] -> Maybe a; last3 [] = Nothing last3 lst@(x:xs) | (null xs) = Just x | otherwise = last3 xs Regards, Philip

Roelof

Max Voit schreef op 9-11-2014 18:04:

...

Hi Roelof,

the problem is, you cannot pattern-match within guards. Guards take predicates.

On your example: "last3 a" pattern-matches the first argument of last3 into a. You may now use a in all further statements. If, however, you want to do pattern matching, it would look like this:

last3::[a]-> Maybe a; last3 [] = Nothing last3 ([a]) = Just a last3 (_:xs) = last3 xs

Notice that the last case will never be executed, as the matching is complete with the first two case.

Max

Am Sun, 09 Nov 2014 17:38:26 +0100 schrieb Roelof Wobben :

...

Hello,

Im trying to find several answers to problem1 of the 99 haskell problems. Find the last item in a list.

Now im trying to find a guarded solution. So far I have this:

last3::[a]-> Maybe a; last3 a | [] = Nothing | ([a]) = Just a | (_:xs) = last3 xs

But I see this error messages :

src/Main.hs@10:8-10:10Not in scope: xs

How to solve this ?

Roelof

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

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