Recursion with closures in Groovy 2.1.9

Question

I am unable to call recursive closures in Groovy 2.1.9

def facRec = {long n->
    return n>1 ? n * facRec(n - 1) : 1
}

I'm getting a TypeMissmatch

Answer 1

When the closure is being defined, it has no idea of the variable facRec as it has not yet been defined...

You can do:

def facRec
facRec = {long n->
    return n>1 ? n * facRec(n - 1) : 1
}

To get around this, or you can wrap the inner into another closure and call the owner of that inner closure (though I would tend to do the above as it is easier to read):

def facRec = {long n->
    { -> n > 1 ? n * owner.call( n - 1 ) : 1 }()
}

It should be noted that both of these will fail for big values of n as you will overflow the stack

You can use trampoline to get round this:

def facRec
facRec = { n, count = 1G ->
    n > 1 ? facRec.trampoline( n - 1, count * n ) : count
}.trampoline()