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我在與Haskell玩耍，我想我會嘗試用它創建一個簡單的編程語言。現在忽略具體的語法;我專注於抽象語法和語義。Haskell中的迷你編程語言
該語言目前應該由整數，整數加法，變量名稱和變量綁定塊組成。
如果使用的變量不在其使用的範圍內，則會引發錯誤。
以下是我目前的進度：
module ProgLang where
import Data.Map as Map
-- Classes
class Runnable d where
run :: (Runnable a) => d -> Map String a -> Either [String] Integer
-- Data
data Name = Name String
deriving (Eq, Ord, Show)
data Add a b = Add a b
deriving (Eq, Ord, Show)
data Block a = Block (Map String a) a
deriving (Eq, Ord, Show)
-- Instances
-- Integers resolve to Right Integer
instance Runnable Integer where
run v _ = Right v
-- For Names
-- look up their expression in the scope, then evaluate
-- if name is out of scope, raise an error
instance Runnable Name where
run (Name n) s = which (Map.lookup n s) where
which Nothing = Left ["Variable not in scope: " ++ n]
which (Just v) = run v s
-- For Addition
-- Run a, Run b, Add their results
-- Raise appropriate errors where necessary
instance (Runnable a, Show a, Runnable b, Show b) => Runnable (Add a b) where
run (Add a b) s = geta (run a s) where
geta (Left es) = Left (es ++ ["In lhs of expression: " ++ show (Add a b)])
geta (Right a') = getb a' (run b s)
getb _ (Left es) = Left (es ++ ["In rhs of expression: " ++ show (Add a b)])
getb a' (Right b') = Right (a' + b')
-- For Blocks
-- Run the block's expression under a new scope
-- (merging the current with the block's scope definition)
instance Runnable a => Runnable (Block a) where
run (Block s' e) s = result $ run e (Map.union s' s) where
result (Left es) = Left (es ++ ["In block: " ++ show (Block s' e)])
result (Right v) = Right v
我使用(Runnable a) => Either [String] a爲run結果。對於錯誤爲Left，對於有效結果爲Right。
下面是示例表達式和他們的預期結果：
-- run 5 Map.empty
-- => Right 5
-- run (Name "a") Map.empty
-- => Left ["Variable not in scope: a"]
-- run (Name "a") (fromList [("a", 6)])
-- => Right 6
-- run (Add 6 3) Map.empty
-- => Right 9
-- run (Add (Name "a") 7) (fromList [("a", 10)])
-- => Right 17
-- run (Block (fromList [("a", 10)]) (Name "a")) Map.empty
-- => Right 10
我正在從GHCI以下錯誤（7.4.1版本）：
progLang.hs:45:53:
Could not deduce (a1 ~ a)
from the context (Runnable a)
bound by the instance declaration at progLang.hs:44:10-41
or from (Runnable a1)
bound by the type signature for
run :: Runnable a1 =>
Block a -> Map String a1 -> Either [String] Integer
at progLang.hs:(45,3)-(47,30)
`a1' is a rigid type variable bound by
the type signature for
run :: Runnable a1 =>
Block a -> Map String a1 -> Either [String] Integer
at progLang.hs:45:3
`a' is a rigid type variable bound by
the instance declaration at progLang.hs:44:19
Expected type: Map String a1
Actual type: Map String a
In the second argument of `union', namely `s'
In the second argument of `run', namely `(union s' s)'
Failed, modules loaded: none.
此錯誤（據我可以說）是由於Block的運行功能。它似乎不喜歡撥打Map.union。
我不確定我做錯了什麼。有任何想法嗎？我應該嘗試一種完全不同的方法來完成這個項目嗎？
在此先感謝。