Groovy String evaluation runtime

Hi I have a String value that is populated runtime and I want to use it to construct another String.

static value= ''
static construct = "${-> value - '/'}"

So when I value = "/http://ift.tt/fyw30c" , construct is equal to "http://ift.tt/fyw30c"

but when I do

static value= ''
static construct = {-> value - '/'}

construct is equals to some closure name. I am wondering what is the purpose of this? Why using closure,GString everything is ok? And why when use only closure nothing happens?