如何正確限定使用隨機monad的類型

Question

我想創建一個數據結構，該結構包含基於以下各項的定義函數：折疊，布爾運算，數字運算（算術和比較）以及基本字符串運算。 我將嘗試發展這些功能類別。 在這里，我收到MonadRandom錯誤。

這是我的代碼：

-- The following language extensions need to be enabled:
-- DeriveDataTypeable, FlexibleInstances, MultiParamTypeClasses

{-# LANGUAGE FlexibleContexts, DeriveDataTypeable, FlexibleInstances, MultiParamTypeClasses, StandaloneDeriving, ExistentialQuantification , GADTs #-}
{-# LANGUAGE RankNTypes #-}

import GenProg
import GenProg.GenExpr
import GenProg.GenExpr.Data
import Data.Generics
import Control.Monad
import Control.Monad.Random
import Data.Fixed
import Data.Typeable
import Data.Data

class IsGenExp t where
      mutate :: (MonadRandom m) => t -> m t
      crossOver :: (MonadRandom m) => t -> t -> m t
      randomGen :: (MonadRandom m) => m t

data GenExp = forall t . IsGenExp t => GenExp t

data IntExp = IntExp Int
data FloatExp = FloatExp Float
data ListExp = ListExp [GenExp]
data ArithExp = Add GenExp GenExp | Sub GenExp GenExp | Mul GenExp GenExp
data CompExp = Eq GenExp GenExp | Lt GenExp GenExp | Gt GenExp GenExp

instance IsGenExp IntExp where
 --  randomGen = (getRandomR (IntExp 1,IntExp 3))
   randomGen = do
    r <- getRandomR (0, 100)
    return r

我得到的錯誤是：

GADT.hs:33:10:
    Could not deduce (Random IntExp) arising from a use of `getRandomR'
    from the context (MonadRandom m)
      bound by the type signature for
                 randomGen :: MonadRandom m => m IntExp
      at GADT.hs:32:4-12
    In a stmt of a 'do' block: r <- getRandomR (0, 100)
    In the expression:
      do { r <- getRandomR (0, 100);
           return r }
    In an equation for `randomGen':
        randomGen
          = do { r <- getRandomR (0, 100);
                 return r }

GADT.hs:33:22:
    Could not deduce (Num IntExp) arising from the literal `0'
    from the context (MonadRandom m)
      bound by the type signature for
                 randomGen :: MonadRandom m => m IntExp
      at GADT.hs:32:4-12
    In the expression: 0
    In the first argument of `getRandomR', namely `(0, 100)'
    In a stmt of a 'do' block: r <- getRandomR (0, 100)
Failed, modules loaded: none.

我該如何解決？

Answer 1

嘗試：

instance IsGenExp IntExp where
   randomGen = do
    r <- getRandomR (0, 100)
    return $ IntExp r

注意：

• r是一個數字
• IntExp r是一個IntExp
• return $ IntExp r是m IntExp單子類型m m IntExp