19 lines
No EOL
457 B
Haskell
19 lines
No EOL
457 B
Haskell
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
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module Unique (guid, set, run, runAt, Unique) where
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import Control.Monad.State (evalState, State, get, put)
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-- Wrapper around State monad.
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newtype Unique a = U { runU :: State Int a }
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deriving (Monad)
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-- Get the next GUID, incrementing the counter.
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guid :: Unique Int
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guid = U $ do n <- get
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put (n + 1)
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return n
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set n = U (put n)
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run = runAt 0
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runAt n x = evalState (runU x) n |