[erlang-questions] read_file -- binary data object

[Donald Steven]

Thanks Joe, much appreciated.

On 12/20/2018 4.49 AM, Joe Armstrong wrote:
> A binary is just an area of memory containing raw bytes.
>
> L = binary_to_list(B) converts the binary B into a list of integers L where
> each integer is in the range 0..255
>
> list_to_binary(L) is the inverse.
>
> A list of small integers of length N takes 16*N bytes of storage (on a
> 64 bit word size erlang)
> A binary of size N takes N+C bytes of storage where C is a small constant
>
> Random access on lists is bad - random access on binaries is far better.
>
> There are primitives to access the individual elements of lists and binaries.
>
> There are also some fancy bit-level things (the binary syntax) so you
> can extract ranges of bits from binaries in a convenient manner.
>
> Cheers
>
> /Joe
>
>
> On Thu, Dec 20, 2018 at 12:48 AM <zxq9@REDACTED> wrote:
>> On 2018年12月19日水曜日 18時40分05秒 JST Donald Steven wrote:
>>> The manual for the function read_file/1 says: Returns {ok, Binary}, where Binary is a binary data object that contains the contents of Filename, or {error, Reason} if an error occurs.
>>>
>>> What is the type of this "binary data object" (a list?).  (I'll want to access individual elements.)
>> It is an Erlang binary.
>>
>>
>> 1> Foo1 = "Something I'll write to disk.".
>> "Something I'll write to disk."
>> 2> Foo2 = <<"Something else I'll write to disk.">>.
>> <<"Something else I'll write to disk.">>
>> 3> Foo3 = <<1,2,3,4>>.
>> <<1,2,3,4>>
>> 4> file:write_file("foo1.txt", Foo1).
>> ok
>> 5> file:write_file("foo2.txt", Foo2).
>> ok
>> 6> file:write_file("foo3", Foo3).
>> ok
>> 7> file:read_file("foo1.txt").
>> {ok,<<"Something I'll write to disk.">>}
>> 8> file:read_file("foo2.txt").
>> {ok,<<"Something else I'll write to disk.">>}
>> 9> file:read_file("foo3").
>> {ok,<<1,2,3,4>>}
>>
>>
>> If the above seems perplexing then check out the docs on Erlang binaries.
>> They are awesome, especially when dealing with binary file formats or network data.
>>
>> http://erlang.org/doc/reference_manual/expressions.html#bit_syntax
>> http://erlang.org/doc/programming_examples/bit_syntax.html
>> http://erlang.org/doc/man/binary.html
>>
>> Also, keep in mind that "object" is a heavily overloaded term in computing.
>> The docs mean "returns a self-contained thingy that can be labeled and carries
>> a type (binary) native to the runtime".
>>
>> -Craig
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