Automatically compute size for <<^var::binary>>

[josevalim]

Elixir and Erlang/OTP versions

All

Operating system

All

Current behavior

Currently it fails to compile.

Expected behavior

It should automatically become <<^var::binary-size(byte_size(^var))>> (if the size is not given).

[josevalim]

If we do <<^var::bitstring>>, then we can compile to <<^var::bitstring-size(bit_size(^var))>>.

[wojtekmach]

I was just writing the following doctest:

iex> url = ReqS3.presign_url(url: "s3://#{bucket}/key1")
iex> "https://#{bucket}.s3.amazonaws.com/key1?X-Amz-Algorithm=AWS4-HMAC" <> _ = url

I've written in such a way that does not reveal bucket. This obviously doesn't work because can't use interpolations inside matches.

This works today:

iex> "https://" <> ^bucket <> ".s3.amazonaws.com/key1?X-Amz-Algorithm=AWS4-HMAC" <> _ = url

WDYT about the following?

iex> "https://#{^bucket}.s3.amazonaws.com/key1?X-Amz-Algorithm=AWS4-HMAC" <> _ = url

In other words, "...#{^x}..." could be rewritten to <<"...", ^x::binary-size(x), "...">>. I'm not sure if folks would even expect this syntax to work but I think it would be super concise in this (admittedly pretty rare) cases and so I thought I'd mention this anyway!

[josevalim]

Interpolations convert to two string, so someone may expect if bucket was an integer or something else, it would be automatically handled, but we can't do the conversion inside patterns, so I am a 👎.