cogent/examples/fib/Fib.cogent

include  <gum/common/iterator.cogent>
 
@ fib_gen --- calculate the next Fibonacci number, unless we're finished.
@ Accumulator contains (n-1)th and nth Fibonacci numbers, and n in third place.
@ The accumulator returned by GeneratorResult has the same pattern; no value is returned for Stop / Yield etc.
 
fib_gen : #{acc : (U32, U32, U32), obsv : U32} -> GeneratorResult () () () (U32, U32, U32)
fib_gen #{acc = (n1, n2, n), obsv} =
  if | n == obsv -> ((n1, n2, n), Stop ())
     | else      -> ((n2, n1+n2, n+1), Yield ())
 
@ fib_consume is a verbos no-op.
fib_consume : #{obj : (), acc : (U32, U32, U32), obsv : U32} -> ConsumerResult () () (U32, U32, U32)
fib_consume #{obj, acc, obsv} = (acc, Next)
 
@ fibonacci instantiates Iterate with the triple described above as accumulator, then invokes it.
fibonacci : U32 -> U32
fibonacci n =
   let ((_, fibn, _), _) = iterate #{
       gen = fib_gen,
       cons = fib_consume,
       acc = (0, 1, 1),
       obsv = n
       }
   in fibn