[erlang-questions] How to understanding recursive inside comprehension lists?

[Sverker Eriksson]

How to create all permutations of List:
  Construct all possible lists [H|T]
   
where H is one element of List
    and T is one of all the permutations of the
remaining elements in List.


And the empty list has one permutation; the empty list.


/Sverker

On fre, 2019-08-23 at 19:03 +0700, I Gusti Ngurah Oka Prinarjaya wrote:
> Hi,
> 
> Now I read Joe's book titled Programming Erlang 2nd Edition. I practice some
> functions such as for/3, quicksort/1, pythag/1, and perms/1, and perms/1 is
> the function that hard to understand.
> 
> I understand comprehension lists, I fully understand for/3, I fully understand
> quicksort/1, pythag/1. But it's really hard for me to understand perms/1.
> Please teach me how to read and understand this perms/1 function.
> 
> perms([]) -> [[]];
> perms(List) -> [ [H|T] || H <- List, T <- perms(List--[H]) ].
> 
> Output:
> 1> lib_misc:perms("123").
> ["123","132","213","231","312","321"]
> 
> Please enlightenment
> 
> Thank you
> 
> 
> 
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