| Author |
Message |
< Advanced Erlang/OTP ~ Odd behaviour with tuple data extraction |
| brett hallett |
 Posted: Sun Sep 23, 2007 11:44 pm |
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User
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 aborts !
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),
also 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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| michal |
| Posted: Mon Sep 24, 2007 11:55 am |
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User
Joined: 20 Jul 2006
Posts: 44
Location: London
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When you see error like {badmatch,[{account,"joe",0}]} try to look in you code for patterns that are similar to [{account,"joe",0}]. I can see a following line:
{A,B,C} = L1,
L1 is a list with one element [{account,"joe",0}] and you try to match it against a tuple. Rewrite the code as:
[{A,B,C}] = L1,
Michal |
_________________
http://www.erlang-consulting.com |
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| Mazen |
| Posted: Tue Sep 25, 2007 8:14 am |
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Joined: 20 Jul 2006
Posts: 164
Location: London
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The reason that you have the badmatch is explained by michal, however, the reason that you have to match against a list with 1 element in it instead of just a tuple is because mnesia:dirty_read/2 returns a *list* or records.
I.e. if dirty_read returns [] then you didn't find anything, if your table is a set or ordered_set then you can patternmatch on [Element] = ... or if your table is a bag then you will have many elements in that list.
In your case it is probably a set and thus returns [{account,Name,Cash}] when you read a record.
/Mazen |
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| brett hallett |
| Posted: Wed Sep 26, 2007 12:05 am |
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Joined: 21 Aug 2007
Posts: 21
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There is more to this tuple / list processing than meets the eye !!
Your input(s) have been very useful, I now have a variery of ways which do what I wanted. But it was a struggle getting any useful feedback from Erlang itself ! The 'badarg' error
seems to he Erlangs catchall
Thanks
Brett |
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| francesco |
| Posted: Wed Sep 26, 2007 6:21 am |
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Joined: 07 Jul 2006
Posts: 249
Location: London
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badarg is returned whenever you call a BIF with wrong arguments. badmatch, instead, when pattern matching fails. There are very few error messages in Erlang. Once you get used to them, it becomes fairly easy to spot what is wrong.
Regards,
Francesco |
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