need help understanding $(eval ...) variable expansion

[gk]

Can anyone explain how to assign the value of $(a) in the following makefile?
This is a simplified version of what I am trying to do: I need to perform 
filtering on the input $(1) so I cannot simply substitue $(1) for $(a) in 
the following example.

# makefile
define macro

a:=$(1)
a-copy:=$(a)
a-reference=$(a)
$(eval a-copy-eval:=$(a))
$(eval a-reference-eval=$(a))

foo:
        @echo 'a=$$(a)'
        @echo 'a-copy=$$(a-copy)'
        @echo 'a-reference=$$(a-reference)'
        @echo 'a-copy-eval=$$(a-copy-eval)'
        @echo 'a-reference-eval=$$(a-reference-eval)'
endef # XMake_outfileRule

$(eval $(call macro,hello))
# eof

- Greg Keraunen