[Haskell-cafe] Can I check the length in a guarded expresion

[Andy Morris]

You also forgot the `|` and `=` either side of the `otherwise`. So the weird error is because GHC is understanding the second clause to be the application `a otherwise last3 (tail a)`.
> On 09 Nov 2014, at 19:51, Roelof Wobben <r.wobben at home.nl> wrote:
> 
> Hello, 
> 
> Here is the complete solution : 
> 
> last3::[a]-> Maybe a; 
> last3 a 
>   | null a = Nothing
>   | length a == 1 = a 
>   otherwise last3 (tail a)
> 
> And I see these error messages : 
> 
> src/Main.hs at 9:21-10:27
> Couldn't match expected type ‘Bool -> ([a0] -> Maybe a0) -> [a] -> Maybe a’ with actual type
> [a]
> Relevant bindings include a :: [a] (bound at /home/app/isolation-runner-work/projects/75679/session.207/src/src/Main.hs:7:7) last3 :: [a] -> Maybe a (bound at /home/app/isolation-runner-work/projects/75679/session.207/src/src/Main.hs:7:1) The function
> a
> is applied to three arguments, but its type
> [a]
> has none …
> 
> 
> 
> 
> Bob Ippolito schreef op 9-11-2014 19:45:
>> You can do that. If you're getting an error, it's from a mistake on some other line. That said, you shouldn't write it that way. length requires a traversal of the entire list, so if the list is 1000 elements long it will take 1001 steps to compute that result when two steps would do. `length (take 2 a) == 1` would be a constant time way to get the result you want, although likely not the most elegant way to solve this exercise.
>> 
>> Perhaps this is the sort of question better suited for the haskell-beginners mailing list: https://www.haskell.org/mailman/listinfo/beginners <https://www.haskell.org/mailman/listinfo/beginners>
>> 
>> On Sun, Nov 9, 2014 at 10:33 AM, Roelof Wobben <r.wobben at home.nl <mailto:r.wobben at home.nl>> wrote:
>> Hello,
>> 
>> I try to make a guarded solution to find the last item in a list.
>> 
>> I thought I could do something like this
>> 
>> | length a == 1 = a
>> 
>> But then I see a lot of error messages.
>> 
>> Is there another way I can check the length of a list with guards ?
>> 
>> Roelof
>> 
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