Pattern matching and list comprehension with binary - binary

Question

Using list comprehension, elixir allow do pattern matching like that:
iex()> for {a,2,c} = ch <- [{1,2,3},{4,5,6},3,4,5], do: c
[3]
But when I'm trying to do something like that with binary, I fail:
iex()> for << b1::size(2), b2::size(3), b3::size(3) >> = <<ch>> <- 'hello', do: b1
[]
Nevertheless, it matches well when standalone:
<< b1::size(2), b2::size(3), b3::size(3) >> = <<100>>
"d"
iex(282)> b2
4
iex(283)> b1
1
iex(284)> b3
4
It also works well when I pass mattern matching clause as secornd parameter to for:
iex(286)> for ch <- 'hello', << b1::size(2), b2::size(3), b3::size(3) >> = <<ch>>, do: b1
[1, 1, 1, 1, 1]
I'm interested if it possible to do something like first example with binary.

Answer 1

This is what fails:
<<ch>> <- 'hello'
In your very first example you do var <- list and later you try <<var>> <- list which is not the same by all means.
'hello' is the list of integers in the first place. Check this:
[104,101,108,108,111]
#⇒ 'hello'
Kernel.SpecialForms.for/1 is iterating through the list, one by one. One cannot match binary to an integer as is:
<<_::size(2), _::size(3), _::size(3)>> = 101
#⇒ ** (MatchError) no match of right hand side value: 101
And also:
<<ch>> = 101
#⇒ ** (MatchError) no match of right hand side value: 101
The latter code example from your question works because you match to an integer and then explicitly tell to Elixir/Erlang it’s to be treated as binary by wrapping it with << >>:
<<b1::size(2), _::size(3), _::size(3)>> = <<101>>
#⇒ "e"