How can I call user defined method inside repositories function of buildscript function in build.gradle

I define a function in project level build.gradle

def isGood(){
    return true
}

Again in project level, I want to call this function inside repositories function of buildscript function. But isGood cannot be called from here. Why? And how can I do it?

buildscript {
    repositories {

        if(isGood()) {. // isGood cannot be found here. I want to call isGood function here. How can I do that?
            maven { url "https://something" }
        }

    }
    dependencies {

    }
}