Rewrite variable in Erlang

Question

I am playing with records and list. Please, I want to know how to use one variable twice. When I assign any values into variable _list and after that I try rewrite this variable then raising error:

** exception error: no match of right hand side value

-module(hello).
-author("anx00040").

-record(car, {evc, type, color}).
-record(person, {name, phone, addresa, rc}).
-record(driver, {rc, evc}).

-record(list, {cars = [], persons = [], drivers = []} ).

%% API
-export([helloIF/1, helloCase/1, helloResult/1, helloList/0, map/2, filter/2, helloListCaA/0, createCar/3, createPerson/4, createDriver/2, helloRecords/0, empty_list/0, any_data/0, del_Person/1, get_persons/1, do_it_hard/0, add_person/2]).


createCar(P_evc, P_type, P_color) -> _car = #car{evc = P_evc, type = P_type, color = P_color}, _car
  .
createPerson(P_name, P_phone, P_addres, P_rc) -> _person= #person{name = P_name, phone = P_phone, addresa = P_addres, rc = P_rc}, _person
  .
createDriver(P_evc, P_rc) -> _driver = #driver{rc = P_rc, evc = P_evc}, _driver
  .  

empty_list() ->
  #list{}.

any_data() ->
  _car1 = hello:createCar("BL 4", "Skoda octavia", "White"),
  _person1 = hello:createPerson("Eduard B.","[phone]","Kr, 81107 Bratislava1", "8811235"),
  _driver1 = hello:createDriver(_car1#car.evc, _person1#person.rc),

  _car2 = hello:createCar("BL 111 HK", "BMW M1", "Red"),
  _person2 = hello:createPerson("Lenka M","[phone]","Krizn0, 81107 Bratislava1", "8811167695"),
  _driver2 = hello:createDriver(_car2#car.evc, _person2#person.rc),

  _car3 = hello:createCar("BL 123 AB", "Audi A1 S", "Black"),
  _person3 = hello:createPerson("Stela Ba.","[phone]","Azna 20, 81107 Bratislava1", "8811167695"),
  _driver3 = hello:createDriver(_car3#car.evc, _person3#person.rc),

  _list = #list{
    cars = [_car1,_car2,_car3],
    persons = [_person1, _person2, _person3],
    drivers = [_driver1, _driver2, _driver3]},
  _list.


add_person(List, Person) ->
  List#list{persons = lists:append([Person], List#list.persons) }.

get_persons(#list{persons = P}) -> P.

do_it_hard()->
  empty_list(),
  _list = add_person(any_data(), #person{name = "Test",phone = "+421Test", addresa = "Testova 20 81101 Testovo", rc =88113545}),
  io:fwrite("\n"),
  get_persons(add_person(_list, #person{name = "Test2",phone = "+421Test2", addresa = "Testova 20 81101 Testovo2", rc =991135455}))
.

But it raising error when i use variable _list twice:

do_it_hard()->
  empty_list(),
  _list = add_person(any_data(), #person{name = "Test",phone = "+421Test", addresa = "Testova 20 81101 Testovo", rc =88113545}),
  _list =add_person(_list, #person{name = "Test2",phone = "+421Test2", addresa = "Testova 20 81101 Testovo2", rc =991135455}),
  get_persons(_list)
.

Answer 1

In the REPL, it can be convenient to experiment with things while re-using variable names. There, you can do f(A). to have Erlang "forget" the current assignment of A.

1> Result = connect("goooogle.com").
{error, "server not found"}
2> % oops! I misspelled the server name
2> f(Result).
ok
3> Result = connect("google.com").
{ok, <<"contents of the page">>}

Note that this is only a REPL convenience feature. You can't do this in actual code.

In actual code, variables can only be assigned once. In a procedural language (C, Java, Python, etc), the typical use-case for reassignment is loops:

for (int i = 0; i < max; i++) {
  conn = connect(servers[i]);
  reply = send_data(conn);
  print(reply);
}

In the above, the variables i, conn, and reply are reassigned in each iteration of the loop.

Functional languages use recursion to perform their loops:

send_all(Max, Servers) ->
 send_loop(1, Max, Servers).

send_loop(Current, Max, _Servers) when Current =:= Max->
 ok;
send_loop(Current, Max, Servers) ->
 Conn = connect(lists:nth(Current, Servers)),
 Reply = send_data(Conn),
 print(Reply).

This isn't very idiomatic Erlang; I'm trying to make it mirror the procedural code above.

As you can see, I'm getting the same effect, but my assignments within a function are fixed.

As a side note, you are using a lot of variable names beginning with underscore. In Erlang this is a way of hinting that you will not be using the value of these variables. (Like in the above example, when I've reached the end of my list, I don't care about the list of servers.) Using a leading underscore as in your code turns off some useful compiler warnings and will confuse any other developers who look at your code.

Answer 2

In some situations it is convenient to use use SeqBind:

SeqBind is a parse transformation that auto-numbers all occurrences of these bindings following the suffix @ (creating L@0, L@1, Req@0, Req@1) and so on.

Simple example:

...
-compile({parse_transform,seqbind}).
...

List@ = lists:seq(0, 100),
List@ = lists:filter(fun (X) -> X rem 2 == 0 end, List@)
...

Answer 3

I used google...

Erlang is a single-assignment language. That is, once a variable has been given a value, it cannot be given a different value. In this sense it is like algebra rather than like most conventional programming languages.

http://www.cis.upenn.edu/~matuszek/General/ConciseGuides/concise-erlang.html