FunListMonad

FunList as Monad

Fpiglet comes with predefined standard 'non-deterministic calculations' monad for functional lists (FunList) defined in:

fpig.funlist.functions.FunListMonad

Below is an example of non-deterministic calculation. Consider a simple game where coin is tossed, heads results in player getting one dollar and tails results in loss of one dollar. What is the likelihood of player being 2 dollars or more ahead after 4 tosses?

Closure toss = { x-> e(x+1) << e(x-1) << empty()} //two element list with (x+1) and (x-1), +1 represents heads (gain), -1 represents tails (loss)

def after4 = b(toss) << b(toss) << b(toss) << b(toss) << f([0])
def res = (length << filter (LARGER(1)) <<after4) / (length << after4)
assert res + 0.01 > 5/16 && res - 0.01 < 5/16 

In the above example static curried function b (same as bind) is defined in FpigBase and applies to all monads registered with Fpiglet. f maps List to Functional List and since FunList is a monad we could have wrote:

MonadDescription m = FunListMonad.instance
m.pure(0)  //same as f([0])

Note that the direction is right to left, which is the same as function composition (which is actually what << is).