Programming :: Capture The Stdout Of A Heredoc In A Variable?

Why doesn't this work?

Code:
cat | myvar=$(</dev/stdin) <<EOS
This is some content
EOS
echo "$myvar"

i have also tried several variants of cat in place of the redirection.

Code:
$(cat -)
$(cat)
$(cat /dev/stdin)

None of the variants print "This is some content"

I have a version that works:

Code:
myvar=$(cat <<EOS
This is some content
EOS
)
echo "$myvar"

but i don't like the syntax and it doesn't play nice in my editor with code folding.