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  1. module Main where
  2. import Data.Map as Map
  3.  
  4. data Runnable
  5.   = Name String
  6.   | Add Runnable Runnable
  7.   | Block (Map String Runnable) Runnable
  8.   | I Integer
  9.   deriving (Eq, Ord, Show)
  10.  
  11. run :: Runnable -> Map String Runnable -> Either [String] Integer
  12. --  Instances
  13. --  Integers resolve to Right Integer
  14. run (I v) _ = Right v
  15. --  For Names
  16. --    look up their expression in the scope, then evaluate
  17. --    if name is out of scope, raise an error
  18. run (Name n) s = which (Map.lookup n s) where
  19.   which Nothing = Left ["Variable not in scope: " ++ n]
  20.   which (Just v) = run v s
  21. --  For Addition
  22. --    Run a, Run b, Add their results
  23. --    Raise appropriate errors where necessary
  24. run (Add a b) s = geta (run a s) where
  25.   geta (Left es) = Left (es ++ ["In lhs of expression: " ++ show (Add a b)])
  26.   geta (Right a') = getb a' (run b s)
  27.   getb _ (Left es) = Left (es ++ ["In rhs of expression: " ++ show (Add a b)])
  28.   getb a' (Right b') = Right (a' + b')
  29. --  For Blocks
  30. --    Run the block's expression under a new scope
  31. --      (merging the current with the block's scope definition)
  32. run (Block s' e) s = result $ run e (Map.union s' s) where
  33.   result (Left es) = Left (es ++ ["In block: " ++ show (Block s' e)])
  34.   result (Right v) = Right v
  35.  
  36. instance Num Runnable where
  37.   (+) = undefined
  38.   (-) = undefined
  39.   (*) = undefined
  40.   negate = undefined
  41.   signum = undefined
  42.   fromInteger = I
  43.  
  44. main = mapM_ print
  45.   [ run 5 Map.empty
  46.   , run (Name "a") Map.empty
  47.   , run (Name "a") (fromList [("a", 6)])
  48.   , run (Add 6 3) Map.empty
  49.   , run (Add (Name "a") 7) (fromList [("a", 10)])
  50.   , run (Block (fromList [("a", 10)]) (Name "a")) Map.empty
  51.   ]
Success #stdin #stdout 0.01s 3584KB
comments (?)
stdin 
Standard input is empty
stdout 
Right 5
Left ["Variable not in scope: a"]
Right 6
Right 9
Right 17
Right 10
https://ideone.com/9UbC5
language:
Haskell (ghc 8.4.4)
created:
12 years ago
visibility:
public

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