[erlang-questions] What does it mean when an evaluated expression is a list that contains the pipe character?

[zxq9]

On 2015年2月25日 水曜日 00:53:01 zxq9 wrote:
> On 2015年2月24日 火曜日 08:37:38 Cole Fichter wrote:
> > I came across the example:
> > > [1,2|3].
> > 
> > [1,2|3]
> > 
> > I understand that the pipe character, |, can be used to append an item to
> > the head of the list or to pop an item off the list in a pattern match.
> > 
> > But what exactly is happening in the example above? Why does the evaluated
> > expression still contain the pipe?
> > 
> > My best guess is that the final expression results in a match
> > specification
> > that could be used in a pattern match. However, that seems strange too
> > because if so, we'd be popping two items off the list, which should be
> > illegal.
> > 
> > Can someone shed some light?
> 
> It is entirely acceptable to match an arbitrary number of items off the top
> of a list, provided there are at least as many in the list:
> 
> 1> SomeList = [1, 2, 3, 4, 5, 6].
> [1,2,3,4,5,6]
> 2> [A, B, C | Rest] = SomeList.
> [1,2,3,4,5,6]
> 3> A.
> 1
> 4> B.
> 2
> 5> C.
> 3
> 6> Rest.
> [4,5,6]

As an addendum, since someone mentioned improper lists...

7> L = [1,2,3].
[1,2,3]
8> [1, 2 | 3] = L.
** exception error: no match of right hand side value [1,2,3]
9> [1, 2 | [3]] = L.
[1,2,3]

Pay close attention to those last two...