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Today's Topics:
1. Re: Are these solution the Haskell way ? (Daniel P. Wright)
2. Re: Are these solution the Haskell way ? (Frerich Raabe)
----------------------------------------------------------------------
Message: 1
Date: Wed, 13 May 2015 16:08:59 +0900
From: "Daniel P. Wright" <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Are these solution the Haskell way ?
Message-ID:
<CAP=piqfz-x7k0cix+v6sqnz6se4ject2d_nlswvh4evsj1u...@mail.gmail.com>
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Think about what the possible values of "xs" might be, and trace through
the next call to "plusplus xs ys".
It would help if I didn't name the variables stupidly... here is a slightly
better version:
plusplus [] ys = ys
plusplus (x:xs) ys = x : plusplus xs ys
If it helps, try tracing through the steps required to evaluate "plusplus
[1, 2] [3, 4]" manually (using a pen and paper, not on the computer)
2015-05-13 15:55 GMT+09:00 Roelof Wobben <[email protected]>:
> Thanks,
>
> So the answer is x and the rest of xs and ys.
> How do x then get added to ys.
>
> Roelof
>
> Daniel P. Wright schreef op 13-5-2015 om 8:52:
>
> Ah, that's just a small syntactic issue -- in Haskell, operators are infix
> (go between the arguments) by default, but named functions are not. So you
> would have to write it:
>
> plusplus [] xs = xs
> plusplus (x:xs) ys = x : plusplus xs ys
>
>
> 2015-05-13 15:39 GMT+09:00 Roelof Wobben <[email protected]>:
>
>> Thanks,
>>
>> If I re-implement it like this :
>>
>> plusplus :: [a] -> [a] -> [a]
>> plusplus [] (xs) = xs
>> plusplus (x:xs) yx = x : xs plusplus yx
>>
>> main = print $ ["a","b"] plusplus ["c","d"]
>>
>>
>> I see this error appear :
>>
>> src/Main.hs@3:26-3:40
>> Couldn't match expected type ?([a0] -> [a0] -> [a0]) -> [a] -> [a]? with
>> actual type
>> [a]
>> Relevant bindings include yx :: [a] (bound at
>> /home/app/isolation-runner-work/projects/112825/session.207/src/src/Main.hs:3:17)
>> xs :: [a] (bound at
>> /home/app/isolation-runner-work/projects/112825/session.207/src/src/Main.hs:3:13)
>> x :: a (bound at
>> /home/app/isolation-runner-work/projects/112825/session.207/src/src/Main.hs:3:11)
>> plusplus :: [a] -> [a] -> [a] (bound at
>> /home/app/isolation-runner-work/projects/112825/session.207/src/src/Main.hs:2:1)
>> The function
>> xs
>> is applied to two arguments, but its type
>> [a]
>> has none ?
>>
>> So for me not a aha moment. I was hoping I would get it
>>
>> Roelof
>>
>> Daniel P. Wright schreef op 13-5-2015 om 8:27:
>>
>> Hi Roelof,
>>
>> If you don't consider it cheating (and I suggest you shouldn't, having
>> had a stab at the answers), you will find great enlightenment looking at
>> how these functions are *actually* implemented in the wild. Did you know
>> that there is a "source" link for each function on Hackage? By clicking on
>> that, for your first example, you can compare the actual implementation of
>> (++) with your "plusplus" function:
>>
>>
>> http://hackage.haskell.org/package/base-4.8.0.0/docs/src/GHC-Base.html#%2B%2B
>>
>> (By the way, it's that function in particular that gave me one of my
>> first "Aha!" moments in Haskell... it's quite beautiful in its simplicity).
>>
>> One thing with viewing the source on Hackage is that sometimes it can
>> be a little more confusing than it needs to be for the sake of efficiency.
>> A really good source for good, readable examples of Prelude functions in
>> Haskell is the Haskell Report:
>>
>> https://www.haskell.org/onlinereport/standard-prelude.html
>>
>> (In this case, though, the implementation is the same).
>>
>> Having a shot at defining these library functions yourself, as you have
>> done, and then comparing your version with the "official" version in the
>> prelude is a great way to learn good style!
>>
>> -Dani.
>>
>> 2015-05-13 15:10 GMT+09:00 Roelof Wobben <[email protected]>:
>>
>>> Hello,
>>>
>>> For practising pattern matching and recursion I did recreate some
>>> commands of Data,list.
>>>
>>> My re-implementation of ++ :
>>>
>>> plusplus :: [a] -> [a] -> [a]
>>> plusplus [] [] = [] ;
>>> plusplus [] (xs) = xs
>>> plusplus (xs) [] = xs
>>> plusplus (xs) yx = plusplus' (reverse xs) yx
>>>
>>> plusplus' :: [a] -> [a] -> [a]
>>> plusplus' [] (xs) = xs
>>> plusplus' (x:xs) yx = plusplus' xs (x:yx)
>>>
>>> main = print $ plusplus ["a","b"] ["c","d"]
>>>
>>> my re-implementation of init :
>>>
>>> import Data.Maybe
>>>
>>> -- | The main entry point.
>>> init' :: [a] -> Maybe [a]
>>> init' [] = Nothing
>>> init' [x] = Just []
>>> init' (x:xs) = Just (x:fromMaybe xs (init' xs))
>>>
>>> main = print . init' $ [1,3]
>>>
>>>
>>> my re-implementation of last :
>>>
>>> -- | The main entry point.
>>> last' :: [a] -> Maybe a
>>> last' [] = Nothing
>>> last' [x] = Just x
>>> last' (_:xs) = last' xs
>>>
>>> main = print . last' $ []
>>>
>>> Now I wonder if these solutions are the haskell way ? if not so, how can
>>> I improve them ,
>>>
>>> Roelof
>>>
>>>
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>>>
>>
>>
>>
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Message: 2
Date: Wed, 13 May 2015 09:17:46 +0200
From: Frerich Raabe <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Are these solution the Haskell way ?
Message-ID: <[email protected]>
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Roelof,
since these functions are all pure (i.e. solely defined through their
arguments) you can apply equational reasoning, which means: everytime the
'plusplus' function is called you can replace that call with the definition
of the function without changing the behaviour of the program.
That allows you to just evaluate a call manually, expanding each function
call by hand. I added some extra parentheses to emphasize the functions being
evaluate:
plusplus [1,2,3] [4,5,6] | apply 'plusplus', second
pattern for 'plusplus' matches
== 1 : (plusplus [2,3] [4,5,6]) | "
== 1 : (2 : (plusplus [3] [4,5,6])) | "
== 1 : (2 : (3 : (plusplus [] [4,5,6]))) | apply 'plusplus', first
pattern for 'plusplus' matches
== 1 : (2 : (3 : [4,5,6]) | apply (:) function
== 1 : (2 : [3,4,5,6]) | "
== 1 : [2,3,4,5,6] | "
== [1,2,3,4,5,6] | "
On 2015-05-13 08:55, Roelof Wobben wrote:
> Thanks,
>
> So the answer is x and the rest of xs and ys.
> How do x then get added to ys.
>
> Roelof
>
> Daniel P. Wright schreef op 13-5-2015 om 8:52:
>
>> Ah, that's just a small syntactic issue -- in Haskell, operators are infix
>> (go between the arguments) by default, but named functions are
>> not. So you would have to write it:
>>
>> plusplus [] xs = xs
>> plusplus (x:xs) ys = x : plusplus xs ys
>>
>> 2015-05-13 15:39 GMT+09:00 Roelof Wobben <[email protected]>:
>>
>> Thanks,
>>
>> If I re-implement it like this :
>>
>> plusplus :: [a] -> [a] -> [a]
>> plusplus [] (xs) = xs
>> plusplus (x:xs) yx = x : xs plusplus yx
>>
>> main = print $ ["a","b"] plusplus ["c","d"]
>>
>> I see this error appear :
>>
>> src/Main.hs@3:26-3:40
>> Couldn't match expected type ?([a0] -> [a0] -> [a0]) -> [a] -> [a]? with
>> actual type
>> [a] Relevant bindings include yx :: [a] (bound at
>> /home/app/isolation-runner-work/projects/112825/session.207/src/src/Main.hs:3:17)
>>
>> xs ::
>> [a] (bound at
>> /home/app/isolation-runner-work/projects/112825/session.207/src/src/Main.hs:3:13)
>>
>> x :: a (bound at
>> /home/app/isolation-runner-work/projects/112825/session.207/src/src/Main.hs:3:11)
>>
>> plusplus :: [a] -> [a] -> [a] (bound at
>> /home/app/isolation-runner-work/projects/112825/session.207/src/src/Main.hs:2:1)
>>
>> The function
>> xs is applied to two arguments, but its type
>> [a] has none ?
>>
>> So for me not a aha moment. I was hoping I would get it
>>
>> Roelof
>>
>> Daniel P. Wright schreef op 13-5-2015 om 8:27:
>>
>> Hi Roelof,
>>
>> If you don't consider it cheating (and I suggest you shouldn't, having had
>> a stab at the answers), you will find great enlightenment looking
>> at how these functions are *actually* implemented in the wild. Did you know
>> that there is a "source" link for each function on Hackage? By
>> clicking on that, for your first example, you can compare the actual
>> implementation of (++) with your "plusplus" function:
>>
>> http://hackage.haskell.org/package/base-4.8.0.0/docs/src/GHC-Base.html#%2B%2B
>>
>> [2]
>>
>> (By the way, it's that function in particular that gave me one of my first
>> "Aha!" moments in Haskell... it's quite beautiful in its
>> simplicity).
>>
>> One thing with viewing the source on Hackage is that sometimes it can be a
>> little more confusing than it needs to be for the sake of
>> efficiency. A really good source for good, readable examples of Prelude
>> functions in Haskell is the Haskell Report:
>>
>> https://www.haskell.org/onlinereport/standard-prelude.html [3]
>>
>> (In this case, though, the implementation is the same).
>>
>> Having a shot at defining these library functions yourself, as you have
>> done, and then comparing your version with the "official" version in
>> the prelude is a great way to learn good style!
>>
>> -Dani.
>>
>> 2015-05-13 15:10 GMT+09:00 Roelof Wobben <[email protected]>:
>> Hello,
>>
>> For practising pattern matching and recursion I did recreate some commands
>> of Data,list.
>>
>> My re-implementation of ++ :
>>
>> plusplus :: [a] -> [a] -> [a]
>> plusplus [] [] = [] ;
>> plusplus [] (xs) = xs
>> plusplus (xs) [] = xs
>> plusplus (xs) yx = plusplus' (reverse xs) yx
>>
>> plusplus' :: [a] -> [a] -> [a]
>> plusplus' [] (xs) = xs
>> plusplus' (x:xs) yx = plusplus' xs (x:yx)
>>
>> main = print $ plusplus ["a","b"] ["c","d"]
>>
>> my re-implementation of init :
>>
>> import Data.Maybe
>>
>> -- | The main entry point.
>> init' :: [a] -> Maybe [a]
>> init' [] = Nothing
>> init' [x] = Just []
>> init' (x:xs) = Just (x:fromMaybe xs (init' xs))
>>
>> main = print . init' $ [1,3]
>>
>> my re-implementation of last :
>>
>> -- | The main entry point.
>> last' :: [a] -> Maybe a
>> last' [] = Nothing
>> last' [x] = Just x
>> last' (_:xs) = last' xs
>>
>> main = print . last' $ []
>>
>> Now I wonder if these solutions are the haskell way ? if not so, how can I
>> improve them ,
>>
>> Roelof
>>
>> ---
>> Dit e-mailbericht is gecontroleerd op virussen met Avast antivirussoftware.
>> http://www.avast.com [4]
>>
>> _______________________________________________
>> Beginners mailing list
>> [email protected]
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners [1]
>>
>> _______________________________________________
>> Beginners mailing list
>> [email protected]
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners [1]
>
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> [5]
>
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> www.avast.com [5]
>
> _______________________________________________
> Beginners mailing list
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> [5]
>
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>
> Links:
> ------
> [1] http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
> [2]
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> [3] https://www.haskell.org/onlinereport/standard-prelude.html
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--
Frerich Raabe - [email protected]
www.froglogic.com - Multi-Platform GUI Testing
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