[erlang-questions] Erlang again!

[Richard O'Keefe]

On Jul 22, 2010, at 11:28 PM, Jilani Khaldi wrote:

> Hi All,
> after a long time, here I am programming in Erlang again...
> I have 2 lists:
> L1=[1,2,3,4,5], L2=[1.1,2.2,3.3,4.4,5.5]
> and I want to have a third list which is the difference of them:
> L3=[
> L2[1] - L1[1],
> L2[2] - L1[2],
> L2[3] - L1[3],
> L2[4] - L1[4],
> L2[5] - L1[5]
> ]
> The result should be
> L3=[0.1,0.2,0.3,0.4,0.5]
> 
> I have tried:
> 
> sub(L2,L1) -> [X2-X1||X1<-L1,X2<-L2].

This says
    for each X1 in L1 {
        for each X2 in L2 {
            include X1 - X2 in the result
        }
    }
just like it does in Haskell and Clean.

Direct method:
   sub([X|Xs], [Y|Ys]) -> [X-Y | sub(Xs, Ys)];
   sub([],     [])     -> [].

Zip then enumerate pairs:
   sub(Xs, Ys) -> [X-Y || {X,Y} <- lists:zip(Xs, Ys)].

Zip with a function other than pairing:
   sub(Xs, Ys) -> lists:zipwith(fun (X, Y) -> X - Y end, Xs, Ys).

One expects a Haskell compiler to expand calls to zip.
One expects an Erlang compiler NOT to, and it doesn't.
There's an EEP proposing that you should be allowed to write

 ** sub(Xs, Ys) -> [X-Y || X <- Xs && Y <- Ys]

but that's not allowed in Erlang yet.