[erlang-questions] "prefix" ++ Rest = Something

[Raimo Niskanen]

On Wed, Oct 25, 2017 at 12:54:59PM +0200, Pierre Fenoll wrote:
> It's the same with binaries:
> 
> <<"a", Rest/binary>> = <<"abc">>
> %%=> Rest = <<"bc">>
> 
> a() -> <<"a">>
> Prefix = <<(a())/binary>>
> PrefixSize = byte_size(Prefix)  %% Compilation fails if size is not provided
> <<Prefix:PrefixSize/binary, Rest/binary>> = <<"abc">>
> %%=> Rest = <<"bc">>

The only time an unspecified size is allowed in the bit syntax is for the
last field in a bit expression.

> 
> Not sure what the differences may be due to compilation v. in the REPL.
> 
> I never understood however why this was "just sugar" or why PrefixSize is
> explicitly needed:
>   in
>       f(Prefix) when is_list(Prefix) ->
>           Prefix ++ Rest = "my_list_thing",
>           Rest.
>   why isn't the compiler able to generate a pattern match? Is it missing
> some concept, some structures?
> What are the missing pieces and what can be done to add them?

    "abc" ++ Rest

is compiled to

    [$a, $b, $c | Rest]

The variable Prefix is a complete list hence there is only a pointer to the
head in runtime, so the only way to append to it is via lists:append/2 i.e
the actual operator erlang:'++'/2, which creates a new list.  So this can
not be described as a pattern.

The visual appearence of "abc"++Rest vs Prefix++Rest is misleading:

   [$a, $b, $c | Rest] vs lists:append(Prefix, Rest)


> 
> Same question with binaries:
>   since we have a "prefix-match of binaries only providing PrefixSize at
> runtime" instruction,
>       * Why don't we have one for lists?
>       * And oh God why do we have to provide that PrefixSize "manually"?
> (binding the variable ourselves, when the compiler could do that itself)
>       * Why isn't suffix-matching of binaries implemented yet? (how
> different to prefix-matching can it be?)
> 
> I could never find the answer to all of these questions.
> WRT binaries my thinking is that they are actually a mix of "bytes" and
> references to binaries,
> making some crucial operations O(log n) instead of O(1)... but prefix match
> exists...
> 
> I really want to be able to write things like:
>     <<"{", Name/binary, "}">> = PathToken
> 
> %%=> Name = <<"id">> given PathToken = <<"{id}">>
> 
> 
> 
> Cheers,
> -- 
> Pierre Fenoll
> 
> 
> On 25 October 2017 at 09:27, Kostis Sagonas <kostis@REDACTED> wrote:
> 
> > On 10/25/2017 08:56 AM, Stefan Hellkvist wrote:
> >
> >> Is it because this syntactic sugar is transformed more or less as a
> >> preprocessing step where the value of Prefix needs to be known,
> >>
> >
> > Yes, that's the reason.  It's a purely _syntactic_ thing.
> >
> > Kostis
> > _______________________________________________
> > erlang-questions mailing list
> > erlang-questions@REDACTED
> > http://erlang.org/mailman/listinfo/erlang-questions
> >

> _______________________________________________
> erlang-questions mailing list
> erlang-questions@REDACTED
> http://erlang.org/mailman/listinfo/erlang-questions


-- 

/ Raimo Niskanen, Erlang/OTP, Ericsson AB