[Haskell-cafe] Thompson's Exercise 9.13

[Hamilton Richards]

At 6:53 PM +0200 2005/4/12, Henning Thielemann wrote:
>On Tue, 12 Apr 2005, Hamilton Richards wrote:
>
>>Here's a solution:
>>
>>>   init :: [a] -> [a]
>>>   init xs = tail (foldr keep [] xs)
>>>     where
>>>     keep :: a -> [a] -> [a]
>>>     keep x []  = [x]
>>>     keep x [y] = [x,x]
>>>     keep x (y:z:zs)  = x:x:y:zs
>
>Nice idea!
>
>One case can be eliminated.
>
>>init :: [a] -> [a]
>>init xs = tail (foldr keep [] xs)
>>   where
>>     keep :: a -> [a] -> [a]
>>     keep x []  = [x]
>>     keep x (_:zs)  = x:x:zs

Yes, much nicer!

Thanks,

--Ham
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Hamilton Richards, PhD           Department of Computer Sciences
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