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Message |
< Advanced Erlang/OTP ~ Processing records extracted from Mnesia problem |
| brett hallett |
 Posted: Wed Sep 19, 2007 5:15 am |
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Joined: 21 Aug 2007
Posts: 21
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Playing with Joe Armstrong's 'Bank' example, I am trying to develop a function the returns all the account records
so I can print a report.
When I execute the folowing :
========== function in bank.erl ===========================
accounts() -> %% returns :{atomic,{atomic,[[{account,"joe",0}],
%% [{account,"richard",19800}],
%% [{account,"brett",4000}]]}}
fun() ->
M = fun() ->
[mnesia:dirty_read(account, Key) || Key <- mnesia:dirty_all_keys(account)]
end,
mnesia:transaction(M)
end.
===================================
4> bank_client:accounts().
{atomic,{atomic,[[{account,"joe",0}],
[{account,"richard",19800}],
[{account,"brett",4000}]]}}
===================================
Which is correct,  , but how do I process the returned result ??
If I set up K ( in the erl shell) , thus :
K = {atomic,{atomic,[[{account,"joe",0}],[{account,"richard",19800}],[{account,"brett",4000}]]}}.
it takes two tuple element extractions to 'get to' the List of accounts for printing purposes,
this seems a bit clumsy !
==================================
eg:
Eshell V5.5 (abort with ^G)
1> K = {atomic,{atomic,[[{account,"joe",0}],[{account,"richard",19800}],[{account,"brett",4000}]]}}.
{atomic,{atomic,[[{account,"joe",0}],
[{account,"richard",19800}],
[{account,"brett",4000}]]}}
2> K1 = element(2,K).
{atomic,[[{account,"joe",0}],
[{account,"richard",19800}],
[{account,"brett",4000}]]}
3> K2 = element(2,K1).
[[{account,"joe",0}],[{account,"richard",19800}],[{account,"brett",4000}]]
What is the appropriate technique to use ???
Thanks. |
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| Mazen |
| Posted: Wed Sep 19, 2007 8:17 am |
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Joined: 20 Jul 2006
Posts: 164
Location: London
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I would suggest patternmatching...
If you know that the value will look like this:
then you would use the following to get the Value:
Code:
{_,{_,Val}} = function_that_returns_that_structure();
If you wan return different structures, use a case clause and patternmatch directly on that. Eg.
Code:
case function_that_returns_different_structures() of
{_,{_,V}} -> V;
{_, V} -> V
end.
On a side note: Are you sure you are getting a list of {atomic, {atomic, ...}} ? Because you are using dirty_reads and you shouldn't get {atomic, _} on a dirty read. Second, if you are using dirty read, why doing so in a transaction? It defeats the purpose doesn't it?
GL,
/Mazen |
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| brett hallett |
| Posted: Wed Sep 19, 2007 10:11 pm |
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Joined: 21 Aug 2007
Posts: 21
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Thanks for your reply,
1) yes I do get the data returned in the format show,
I had tried a number of variations of what you suggest -- without success !
2) I did not know that ditry_reads could operate
'outside' the mnesia:transaction framework. Thanks fror that very useful info.
I'm still feeling my way with Erlang , a very interesting language indeed. |
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| brett hallett |
| Posted: Sat Sep 22, 2007 4:51 am |
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Joined: 21 Aug 2007
Posts: 21
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Following mazen's information I changed my dirty_reads as per accounts() below, and then started expirementing
with possible printing options -- now some confusion beging !.
======================================
accounts() -> %% returns :{atomic,{atomic,[[{account,"joe",0}],
%% [{account,"richard",19800}],
%% [{account,"brett",4000}]]}}
M2 = [mnesia:dirty_read(account, Key)
|| Key <- mnesia:dirty_all_keys(account)],
print(M2),
print2(M2).
%%% ============
print( [L1 | L ]) -> %% {account,"joe",0}
io:fwrite("print1:",[]),
io:fwrite('~p ~n', L1),
print(L);
print([]) ->
io:fwrite("** end accounts list 1 ***~n", []),
ok.
%%% ============
print2( [L1 | L ]) ->
io:fwrite("print2:",[]),
io:fwrite('~p ~n', L1),
print2(L);
print2([]) ->
io:fwrite("** end accounts list 2 ***~n", []),
ok.
=======================================
shell output is:
c(bank).
{ok,bank}
53> bank_client:accounts().
print1:{account,"joe",0}
print1:{account,"richard",19800}
print1:{account,"brett",4000}
** end accounts list 1 ***
print2:{account,"joe",0}
print2:{account,"richard",19800}
print2:{account,"brett",4000}
** end accounts list 2 ***
{aborted,{badarg,ok,[],infinity,mnesia}}
print & print2 output is the same, as expected, however when I change print2 thus :
%%% ============
print2( [L1 | L ]) ->
io:fwrite("print2:",[]),
{A,B,C} = L1, %%%% new
io:fwrite('~p ~n', L1),
io:fwrite("Account : ~s Name: ~s Balance: ~s ~n", [A, B, C]), %%%% new
print2(L);
print2([]) ->
io:fwrite("** end accounts list 2 ***~n", []),
ok.
===============================
the fwrite(s) just stuff up and abort !
I've tried MANY different formats ( ~p, ~s, ~w etc ) without sucess, but the most annoying thing
is that a previously working fwrite
io:fwrite('~p ~n', L1),
does not work now ???
I should get
print2:{account,"joe",0}
, but only 'print2:' appears.
I assume the 'badmatch' below is something to do with my format statement, but I'm
not really sure about that at all !!!
===============================
c(bank).
{ok,bank}
55> bank_client:accounts().
print1:{account,"joe",0}
print1:{account,"richard",19800}
print1:{account,"brett",4000}
** end accounts list 1 ***
print2:
=ERROR REPORT==== 22-Sep-2007::14:23:40 ===
Error in process <0.429.0> with exit value: {{badmatch,[{account,"joe",0}]},[{bank,print2,1},{bank_server,do_call,1},{bank_server,input_handler,1}]}
true
===============================
Any ideas appreciated. |
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