[erlang-patches] Scan "=<<" as something useful

Anthony Ramine <>
Wed May 1 15:14:58 CEST 2013

Hello Raimo,

I've given some more thought about that subject and whether the fix could surprise people if they ever did a typo.

1/ "A=<<<1>>" ("A =< << 1 >>") instead of "A=<<1>>" ("A = << 1 >>")

* If A is not bound, this is a compilation error. The intended code would have compiled fine.
* If A is bound, the intended code is to crash if A isn't <<1>>, that kind of quick and dirty check is usually to fail early and found not nested in a function body; the typo is a comparison expression and the compiler will complain its return value is not used.
* If A is bound and the expression's return value used, no compilation error or warning will be triggered but Dialyzer will complain about a binary/boolean type mismatch.

2/ "A=<<1>>" ("A = << 1 >>") instead of "A=<<<1>>" ("A =< << 1 >>")

* If A isn't bound, code compiles just fine. The intended code would have failed with a compilation error.
* If A is bound, no compilation error but Dialyzer will complain of a binary/boolean type mismatch.

So we are left with two cases where the compiler will say nothing, but Dialyzer will.


Anthony Ramine

Le 29 avr. 2013 à 15:32, Raimo Niskanen a écrit :

> On Mon, Apr 29, 2013 at 03:11:44PM +0200, Anthony Ramine wrote:
>> I don't understand how is that a typo, as "A=<<<1>>" is valid syntax with or without my patch.
> ...i misread the old arguments; rather that intending to write "A =< << 1 >>"
> without spaces as "A=<<<1>>" and misspelling to "A=<<1>>" would give no compiler
> error and unexpected runtime behaviour...
>> -- 
>> Anthony Ramine
>> Le 29 avr. 2013 à 15:05, Raimo Niskanen a écrit :
>>> One final straw back then was this: the typo "A=<<<1>>" accidentally
>>> becomes valid syntax with the proposed change.
> -- 
> / Raimo Niskanen, Erlang/OTP, Ericsson AB
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