Safe recursion with monads.

@unroll
def fact(me, n):
    return (Thunk(1)
        if not n
        else me(n-1).map(lambda x: x * n))

$ pip install thunctor

The thunk monad allows for a lazy value to be constructed whose manipulations will be delayed. These values can then be unrolled without exhausting the callstack.

• Thunk : a → Thunk a
• Thunk#bind : Thunk a → (a → Thunk b) → Thunk b
• Thunk#map : Thunk a → (a → b) → Thunk b

A thunk returning function can be transformed to a normal function using unroll.

• unroll : ((*x, **y) → Thunk a) → (*x, **y) → a