[Agda] Type Mismatch in with

[Andreas Abel]

Ashish, you need

   a ≟ x   (type a \?= x)

instead of  a ≡ x  or  Dec (a ≡ x).

Cheers, Andreas


On 05.11.2015 21:19, Ashish Mishra wrote:
> Hi Harley,
> Thanks for the swift reply, Correct me if i am wrong ,
> The issue here is that the typechecker did not find the type of " yes _
> " as Set which it is looking for , although i am not sure why , because
> the type of yes _ is indeed a Set as per the definition of Dec which is
> of type Set.
>
> P.S. The parameter to *with* needs to be a proposition if I am correct
>   so I don't see Dec ( = a x )  as the issue , moreover replacing it by
> (a = x  ) still gives the same error.
>
> Thanks
>
>
>
>
>
> On Fri, Nov 6, 2015 at 1:02 AM, Harley Eades III <harley.eades at gmail.com
> <mailto:harley.eades at gmail.com>> wrote:
>
>     Hi, Ashish.
>
>     It looks like the problem is the ‘Dec(_=_ a x)’
>
>     I think this should be 'a = x’ instead.  The former is the type used
>     to show
>     that ‘_=_’ is decidable, and so the type of yes becomes Set.
>
>     Very best,
>     Harley
>
>>     On Nov 5, 2015, at 2:18 PM, Ashish Mishra
>>     <ashish123.mishragkp at gmail.com
>>     <mailto:ashish123.mishragkp at gmail.com>> wrote:
>>
>>     Hi All,
>>
>>     I am new to agda programming, please help me understanding what is
>>     wrong here in the function to get an element from the list if present.
>>
>>     /  open import Relation.Nullary.Core/
>>     /  lookupelem : { A : Set } ( a : A ) → List' A → Maybe A /
>>     /  lookupelem a [] = nothing/
>>     /  lookupelem a ( x :: xs ) with Dec( _≡_ a x )/
>>     /  lookupelem a ( x :: xs ) | yes _ = just a /
>>     /  lookupelem a ( x :: xs ) | no _ = lookupelem a xs /
>>
>>     The type checker throws an error -
>>
>>     *Type mismatch*
>>     *when checking that the pattern yes _ has type Set*
>>
>>     where List' is defined same as List from the std-lib , I cant see
>>     where the problem is
>>
>>     Thanks :)
>>
>>     --
>>     Regards,
>>     Ashish Mishra
>>     Graduate Student,
>>     Computer Science and Automation Department,IISc
>>     Cell : +91-9611194714
>>     Mailto : ashishmishra at csa.iisc.ernet.in
>>     <mailto:ashishmishra at csa.iisc.ernet.in>
>>
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>
>
>
> --
> Regards,
> Ashish Mishra
> Graduate Student,
> Computer Science and Automation Department,IISc
> Cell : +91-9611194714
> Mailto : ashishmishra at csa.iisc.ernet.in
> <mailto:ashishmishra at csa.iisc.ernet.in>
>
>
>
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-- 
Andreas Abel  <><      Du bist der geliebte Mensch.

Department of Computer Science and Engineering
Chalmers and Gothenburg University, Sweden

andreas.abel at gu.se
http://www2.tcs.ifi.lmu.de/~abel/