Re: [Haskell-beginners] another type problem

Why not just? ================== init' [] = Nothing init' xs = Just xs ================== Meets your type sig and is also has a time complexity of O(1) instead of O(n) which will be the time complexity in the solution involving fromMaybe. Maybe I'm missing something. Perhaps it'd help us help you if you'd be a bit more clear on what you want to achieve. On Tue, May 12, 2015 at 9:01 PM, Sumit Sahrawat, Maths & Computing, IIT (BHU) wrote:

On 12 May 2015 at 20:14, Roelof Wobben wrote:

...

Thanks,

The right solution was this :

init' (x:xs) = Just (x:fromMaybe xs (init' xs))

if I understand it right x:fromMaybe xs takes care that from xs the just or Nothing is removed ?

Trying it out in ghci,

fromMaybe [1,2] Nothing == [1,2] fromMaybe [1,2] Just [3] == [3]

It seems like that should indeed work.

Roelof

...

Alexey Shmalko schreef op 12-5-2015 om 16:33:

Try fromMaybe [1]. Examples

Prelude Data.Maybe> fromMaybe [] (Just [1,2,3]) [1,2,3] Prelude Data.Maybe> fromMaybe [1,2,3] Nothing [1,2,3]

[1]: https://hackage.haskell.org/package/base-4.8.0.0/docs/Data-Maybe.html#v:from...

On Tue, May 12, 2015 at 5:28 PM Roelof Wobben wrote:

...

Oke,

And how do I do this. Haskell is a difficult one to learn,

Roelof

Alexey Shmalko schreef op 12-5-2015 om 16:24:

Before cons'ing the result of init', you should check whether it's Just or Nothing. What you're doing now is something along the line with 5 : Just 3 -- this won't typecheck.

On Tue, May 12, 2015 at 5:22 PM Roelof Wobben wrote:

...

Brandon Allbery schreef op 12-5-2015 om 16:16:

On Tue, May 12, 2015 at 10:11 AM, Roelof Wobben wrote:

...

I do not understand what you are saying to me.

I know that init produces a Maybe [a] . That is why I did put a Just before it.

You are invoking it again though, and using its result as if it produces [a] instead of Maybe [a].

-- brandon s allbery kf8nh sine nomine associates allbery.b@gmail.com ballbery@sinenomine.net unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net

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What is then the right way to do. I still do not understand what you are trying to make clear to me.

Roelof

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-- Regards

Sumit Sahrawat

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