[erlang-questions] Question about lists:filter

José Valim <>
Wed Aug 28 13:58:32 CEST 2013


In your problem4() function:

*problem4()->lists:max(lists:filter(isPal,cand())).*
*
*
isPal is an atom. It is simply a value. If you want to pass the function
isPal declared above as a function, you need to write:

*problem4()->lists:max(lists:filter(fun isPal/1,cand())).*
*
*
Functions in Erlang are identified by name *and* arity. That's why you need
to explicitly add the arity and use the fun syntax to pass functions around.



*José Valim*
www.plataformatec.com.br
Skype: jv.ptec
Founder and Lead Developer


On Wed, Aug 28, 2013 at 8:54 AM, Nadav Chernin <>wrote:

> Hi, all
>
> I'm learning Erlang with Project Euler.
>
> This is my solution for problem 4.
>
>
> *-module(euler).*
> *-export([problem4/0]).*
> *
> *
> *cand()->[X*Y||X<-lists:seq(100,999),Y<-lists:seq(100,999)].*
> *isPal(X)->integer_to_list(X)==lists:reverse(integer_to_list(X)).*
> *problem4()->lists:max(lists:filter(fun(A)->isPal(A) end,cand())).*
> *
> *
> Before it, my code was:
>
> *-module(euler).*
> *-export([problem4/0]).*
> *
> *
> *cand()->[X*Y||X<-lists:seq(100,999),Y<-lists:seq(100,999)].*
> *isPal(X)->integer_to_list(X)==lists:reverse(integer_to_list(X)).*
> *problem4()->lists:max(lists:filter(isPal,cand())).*
> *
> *
> Difference from my final solution - using isPal instead fun(A)->isPal(A)
> end
>
> But exception occured during execution:
>
> exception error: no function clause matching
>                     lists:filter(isPal,
>
>  [10000,10100,10200,10300,10400,10500,10600,
>
> 10700,10800,10900,11000,11100,11200,11300,
>
> 11400,11500,11600,11700,11800,11900,12000,
>                                   12100,12200,12300,12400,12500,12600|...])
>
> Please, explain me why i can't use function as is, without using lambda
> function.
> I come to Erlang from Haskell, and there i can do this,
>
> Thanks, Nadav
>
>
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