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

Ola Andersson A <>
Thu Feb 17 11:17:33 CET 2011


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