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An odd "non-determinism" example from StackOverflow It is clever, but doesn't make much sense as to how it gets its results
author IBBoard <dev@ibboard.co.uk>
date Sun, 14 Jul 2019 13:44:13 +0100
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98be775c533c An odd "non-determinism" example from StackOverflow
IBBoard <dev@ibboard.co.uk>
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1 module CoinToss where
98be775c533c An odd "non-determinism" example from StackOverflow
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2 -- Based on https://stackoverflow.com/questions/20638893/how-can-non-determinism-be-modeled-with-a-list-monad
98be775c533c An odd "non-determinism" example from StackOverflow
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3 -- Because it only throws "*** Exception: <interactive>:57:1-26: Non-exhaustive patterns in function toss" in gchi
98be775c533c An odd "non-determinism" example from StackOverflow
IBBoard <dev@ibboard.co.uk>
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4 import Control.Monad -- Required for guard, but not mentioned in the example
98be775c533c An odd "non-determinism" example from StackOverflow
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5
98be775c533c An odd "non-determinism" example from StackOverflow
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6 data CoinType = Fair | Biased deriving (Show)
98be775c533c An odd "non-determinism" example from StackOverflow
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7 data Coin = Head | Tail deriving (Eq,Show)
98be775c533c An odd "non-determinism" example from StackOverflow
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8
98be775c533c An odd "non-determinism" example from StackOverflow
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9 toss Fair = [Head, Tail]
98be775c533c An odd "non-determinism" example from StackOverflow
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10 toss Biased = [Head, Head]
98be775c533c An odd "non-determinism" example from StackOverflow
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11
98be775c533c An odd "non-determinism" example from StackOverflow
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12 pick = [Fair, Biased]
98be775c533c An odd "non-determinism" example from StackOverflow
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13
98be775c533c An odd "non-determinism" example from StackOverflow
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14 experiment = do
98be775c533c An odd "non-determinism" example from StackOverflow
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15 coin <- pick -- Pick a coin at random (Non-determinism takes both coins)
98be775c533c An odd "non-determinism" example from StackOverflow
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16 result <- toss coin -- Toss it, to get a result (Non-determinism joins all possible toss results in a single list)
98be775c533c An odd "non-determinism" example from StackOverflow
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17 guard (result == Head) -- We only care about results that come up Heads (Non-determinism does something weird and inexplicable)
98be775c533c An odd "non-determinism" example from StackOverflow
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18 return coin -- Return which coin was used in this case (Magic has happened here and we've suddenly gone from "Head, Tail, Head, Head" to "Fair, Biased, Biased")
98be775c533c An odd "non-determinism" example from StackOverflow
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19
98be775c533c An odd "non-determinism" example from StackOverflow
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20 part_experiment = do
98be775c533c An odd "non-determinism" example from StackOverflow
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21 coin <- pick -- Pick a coin at random
98be775c533c An odd "non-determinism" example from StackOverflow
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22 result <- toss coin -- Toss it, to get a result
98be775c533c An odd "non-determinism" example from StackOverflow
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23 return result -- Return the intermediary results
98be775c533c An odd "non-determinism" example from StackOverflow
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24
98be775c533c An odd "non-determinism" example from StackOverflow
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25 part_experiment_2 = do
98be775c533c An odd "non-determinism" example from StackOverflow
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26 coin <- pick -- Pick a coin at random
98be775c533c An odd "non-determinism" example from StackOverflow
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27 result <- toss coin -- Toss it, to get a result
98be775c533c An odd "non-determinism" example from StackOverflow
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28 return guard (result == Head) -- Return the intermediary results
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29
98be775c533c An odd "non-determinism" example from StackOverflow
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30 -- Load the module and then run "experiment" and you get [Fair,Biased,Biased], which apparently shows that there's a 2/3 chance it's a biased coin