[erlang-questions] Fwd: [erlang-bugs] Incorrect scope for a variable

Raimo Niskanen <>
Thu Feb 17 12:05:06 CET 2011


On Thu, Feb 17, 2011 at 11:17:33AM +0100, Ola Andersson A wrote:
> Are you sure about the example?
> 
> > > 1> {case 1 of X -> X end, case 2 of X -> X; _ -> 4 end}.
> > > ** exception error: no match of right hand side value 1
> 
> I get:
> Erlang R13A (erts-5.7) [smp:2:2] [rq:2] [async-threads:0]
> 
> Eshell V5.7  (abort with ^G)
> 1> case 1 of X -> X end, case 2 of X -> X; _-> 4 end.
> 4

X was exported by the line above so now X is bound.

> 2> {case 1 of X -> X end, case 2 of X -> X; _-> 4 end}.
> {1,4}

Therefore that succeeded.

Try
 1b> X.
 1c> f(X).
in between and watch the difference.

> 3> 
> 
> Another thing that I find strange is that if I use the above constructs in
> a module I get a warning in the first case but not in the second.
> 
> 8> c(warn).   
> ./warn.erl:12: Warning: variable 'X' exported from 'case' (line 7)
> {ok,warn}
> 
> I think I should get a warning for both constructs.
> 
> This export from case thing is something I have never liked.
> I know that there are different opinions about this but I would prefer
> a compile error for using the variable X outside of the first case 
> statement. 
> /OLA.
> 
> > ---------- Forwarded message ----------
> > From: Joe Armstrong <>
> > Date: Thu, Feb 17, 2011 at 9:18 AM
> > Subject: Re: [erlang-bugs] Incorrect scope for a variable
> > To: Alexander Demidenko <>
> > 
> > 
> > On Thu, Feb 17, 2011 at 8:54 AM, Alexander Demidenko < 
> > > wrote:
> > 
> > > Dear friends!
> > >
> > > Let me explain with an synthetic example.
> > >
> > > Erlang R13B04 (erts-5.7.5) [source] [smp:2:2] [rq:2] 
> > [async-threads:0] 
> > > [kernel-poll:false] Eshell V5.7.5  (abort with ^G)
> > >
> > > 1> {case 1 of X -> X end, case 2 of X -> X; _ -> 4 end}.
> > > ** exception error: no match of right hand side value 1
> > >
> > >
> > This is not an error. Try this:
> > 
> > > {case 1 of X -> X end, case 2 of Y -> Y end}.
> > {1,2}
> > 
> > In evaluating {A, B}
> > 
> > any bindings created in A or B are merged, and the common 
> > value propagated to the next lines of code.
> > 
> > In your example
> > 
> > {case 1 of X -> X end ,...}
> > 
> > evaluates to {1, ...} and has the side effect of binding X to 1
> > 
> > {... case 2 of X -> X end}
> > 
> > 
> > evaluates to {, ... 2} and has the side effect of binding X to 2
> > 
> > Now you try to merge the bindings X=1 and X=2 which fails
> > 
> > 
> > 
> > >
> > > opposite successful example with bit change:
> > > 2> f().
> > > ok
> > > 3> begin X=1, {case 1 of X -> X; _ -> 3 end, case 2 of X -> 
> > X; _ -> 4 
> > > 3> end}
> > > end.
> > > {1,4}
> > >
> > >
> > You never bind X inside the tuple, so there is no merging
> > 
> > 
> > 
> > > In first example, erlang assume X=2 in second 'case' (why?) 
> > and trow 
> > > exception before return tuple.
> > >
> > >
> > 
> > 
> > > I'm think this is incorrect behavior. Isn't it?
> > >
> > >
> > No - though on first sight it looked like bug, so I had to think a bit
> > 
> > /Joe
> > 
> > 
> > > --
> > > ---------------------------------------------
> > > With best regards,
> > > Alexander.
> > >
> > > ________________________________________________________________
> > > erlang-bugs (at) erlang.org mailing list.
> > > See http://www.erlang.org/faq.html
> > > To unsubscribe; mailto:
> > >
> > >
> > 
> ________________________________________________________________
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-- 

/ Raimo Niskanen, Erlang/OTP, Ericsson AB


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