[erlang-questions] Basic Erlang case question

[Lev Walkin]

Alexander Lamb wrote:
> Hello List,
> 
> Here is a very simple code to search if a supplied password is correct  
> or not:
> 
> 				F1 = fun() -> mnesia:match_object(systems,  
> #system_info{full_attribute = {System_Name,admin_password}, _ = '_'},  
> read) end,
> 				case Password of
> 					[] -> MD5_Password = [];
> 					_Any -> MD5_Password = erlang:md5(Password)
> 				end,
> 				case mnesia:transaction(F1) of
> 					{atomic, []} 								  -> {error, unknown_system};
> 					{atomic, [#system_info{data = MD5_Password}]} -> {ok, password};
> 					{atomic, _}									  -> {error, bad_password};
> 					{aborted, Reason}							  -> {error, Reason}
> 				end
> 
> 
> 
> Basically, if supplied Password is empty, the MD5_Password must be  
> empty. If not, I encrypt the Password to compare it with stored version.
> 
> My problem is that it compiles with a warning:
> 
> ./cs_systems.erl:173: Warning: variable 'MD5_Password' exported from  
> 'case' (line 167)
> 
> Why? Obviously I am trying to match the password in the record I found  
> with the password I supply.


This Erlang "feature" of leaking identifiers out of "case" is
something to be avoided, like goto's in imperative languages.

Instead of doing

	case Foo of
		Bar -> Password = f1(Bar);
		Baz -> Password = f2(Baz)
	end

do this:

	Password = case Foo of
		Bar -> f1(Bar);
		Baz -> f2(Baz)
	end

In your case,

	HashedPassword = case Password of
		[] -> []
		Value -> erlang:md5(Value)
	end


-- 
vlm