Knightian.hs

{-# LANGUAGE QualifiedDo, RebindableSyntax #-}
-- | Knightian names controlling correlation between choices.
module Imp.Examples.Knightian
  ( Three(..)
  , dependent
  , independent
  ) where

import Imp

-- | Three-valued outcome.
data Three = Red | Green | Blue
  deriving stock (Eq, Ord, Show)

-- | Dependent Knightian choices.  Both branches share the same
--   Knightian variable, so the outcomes are correlated.
dependent :: Imp '["a1"] Three
dependent = Imp.do
  x <- flip 0.5
  if x then Imp.do
    y <- knight @"a1"
    Imp.return (if y then Red else Green)
  else Imp.do
    y <- knight @"a1"
    Imp.return (if y then Red else Blue)

-- | Independent Knightian choices.  Both branches have different
--   Knightian variables, so more outcomes are possible.
independent :: Imp '["a1", "a2"] Three
independent = Imp.do
  x <- flip 0.5
  if x then Imp.do
    y <- knight @"a1"
    Imp.return (if y then Red else Green)
  else Imp.do
    y <- knight @"a2"
    Imp.return (if y then Red else Blue)