[erlang-questions] wierd list comprehension outcomes
Richard A. O'Keefe
Fri Jan 30 05:06:07 CET 2015
On 25/01/2015, at 11:28 pm, Roelof Wobben <r.wobben@REDACTED> wrote:
> I follow this book : Erlang programming and I did read chapter 2.
> Now I have to do this exercise :
> C. Recursive list definitions
> L = [A|[2,3]].
> [H|T] = L.
1. L = [A|[2,3]]
Presumably A already has a value. What is it?
1> L = [A|[2,3]].
* 1: variable 'A' is unbound
2> A = 1.
3> L = [A|[2,3]].
I see no problem here.
Some people will tell you that there is a problem because 1 is not a list
and the tail of a list must be a list. They are wrong.
In Erlang, one kind of term is a pair [Head | Tail],
where Head and Tail can be *absolutely any term whatever*.
The programming language Scheme distinguishes clearly between “pairs”
which can be anything and “lists” which are *made from* pairs as
building blocks: a list is empty () or a pair whose tail is a list.
Erlang has a list/1 *type* and an is_list/1 *guard test*.
is_list([H|T]) succeeds *whatever H and T are*.
http://erlang.org/doc/reference_manual/typespec.html says of types that
List :: list(Type) %% Proper list (-terminated)
| maybe_improper_list(Type1, Type2) %% Type1=contents, Type2=termination
| nonempty_improper_list(Type1, Type2) %% Type1 and Type2 as above
| nonempty_list(Type) %% Proper non-empty list
so is_list([[3,2]|1]) succeeds, but [[3,2]|1] does not have type list(T)
for any type T.
Are you confused yet? If not, why not? I’ve always thought that is_list/1 was
a bad idea.
We can say that ALL [_|_] TERMS ARE DOTTED PAIRS = CONS CELLS,
but ONLY SOME CONS CELLS ARE PARTS OF LISTS.
* 2: syntax error before: '[' which I expected to see also [ 1 , 2 , 3]
As the transcript above shows, there is absolutely no problem with this.
The ‘[‘ in question is presumably the first one, which tells me that there was
some junk before this line.
In any case, there is NO WAY that [[3,2]|1] could possibly give you [1,2,3].
3. [H|T] = L.
I see no problem here.
5> [H|T] = L.
* [H|T] = L.
Which I see ** exception error: no match of right hand side value [1,2,3]
You keep on not showing us everything relevant. “no match” tells me that
either H already has a value that is not 1 or T already has a value that
is not [2,3].
where I was expected to see  [ 2, 3]
You could never have seen . If you write [H|T] = [,2,3] _then_ you
will get H =  and T = [2,3] but not otherwise.
You need to be very clear about
- extending a list on the left by a single element “consing” [New_Head | Old_Tail]
- extending a list on the left by a list of many elements “appending” New_Elements ++ Old_Tail.
9>  ++ L.
10> 0 ++ L.
** exception error: bad argument
in operator ++/2
called as 0 ++ [1,2,3]
Finally, the subject line of your message says “weird list COMPREHENSION outcomes”,
but the body of the message contains no list comprehensions whatsoever.
Did you mean “list CONSTRUCTION outcomes”?
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