GStringの評価が遅延されてるように見えるケースの

1.groovy

// プログラミングGROOVY P69
list = ['x']
def gs = "$list ${list[0]}"

assert gs instanceof GString
assert gs == '[x] x'

list[0]='y'
assert gs == '[y] x'

2.groovy

list = ['x', ['a']]
def gs = "$list ${list[0]} ${list[1]}"

assert gs instanceof GString
assert gs == '[x, [a]] x [a]'

list[0]='y'
assert gs == '[y, [a]] x [a]'

list[1]<<'a'
assert gs == '[y, [a, a]] x [a, a]'

list[1]=['b']
assert gs == '[y, [b]] x [a, a]'

3.groovy

list = ['x']
def gs = "$list ${list[0]}"

assert gs instanceof GString
assert gs == '[x] x'

list[0]='y'
assert gs == '[y] x'

list.add(0, 'z')
assert gs == '[z, y] x'

list=['a']
assert gs == '[z, y] x'

4.groovy

list = ['x']
def gs = "${list} ${list.toString()} ${list[0]}"

assert gs instanceof GString
assert gs == '[x] [x] x'

list[0]='y'
assert gs == '[y] [x] x'

5.groovy

class Hoge {
    String value
    public String toString() {
        println 'hoge:' + value
        value
    }
}

def hoge1 = new Hoge()
hoge1.value='hoge1'

def hoge2 = new Hoge()
hoge2.value='hoge2'

def gs = "$hoge1 ${hoge2.toString()}"

hoge1.value='hoge11'
hoge2.value='hoge22'

println '-' * 20
println gs
println '-' * 20

/* ==== 実行結果 ====
hoge:hoge2
--------------------
hoge:hoge11
hoge11 hoge2
--------------------
*/