Question

当我这样做时：

cabal sandbox init
cabal update
cabal install hakaru
cabal repl
λ> :l simple.hs 
λ> sample test []

simple.hs包含：

{-# LANGUAGE MultiParamTypeClasses #-}
import Language.Hakaru.ImportanceSampler
import Control.Monad.State
instance MonadState Int Measure
test :: Measure Int
test = put 1 >> get >>= \i -> return i

我的电脑内存不足。

如何成功使Measure monad成为MonadState的实例（即test以上返回1）？ Measure类型已经是Monad的实例，其中bind和return已定义。是否有一些默认方式我可以用MonadState，put和get来定义lift＆＃39; s bind和return使它工作？我试过了：

get = lift   get
put = lift . put

但是我无法解决（变压器？）类型：

simple.hs:6:9:
    Couldn't match type ‘t0 m0’ with ‘Measure’
    Expected type: Measure Int
      Actual type: t0 m0 Int
    In the expression: lift get
    In an equation for ‘get’: get = lift get

simple.hs:7:9:
    Couldn't match type ‘t1 m1’ with ‘Measure’
    Expected type: m1 () -> Measure ()
      Actual type: m1 () -> t1 m1 ()
    In the first argument of ‘(.)’, namely ‘lift’
    In the expression: lift . put

Answer 1

Measure已按以下方式定义：

newtype Measure a = Measure { unMeasure :: [Cond] -> Sampler (a, [Cond]) }

您可以看到没有地方存储您的Int，因此您无法将其设为MonadState的正确实例。如果您想将Measure扩展为MonadState，可以使用StateT monad transformer：

test :: StateT Int Measure Int
test = put 1 >> get >>= \i -> return i

这里发生了什么？ StateT s是一个monad转换器，可让您将State s monad与任何其他monad组合在一起（在此示例中为Measure）

Answer 2

最终为我工作的确切代码是：

import Language.Hakaru.ImportanceSampler
import Language.Hakaru.Distribution
import Control.Monad.State
import System.IO.Unsafe (unsafePerformIO)

test1 :: StateT Int Measure Int 
test1 = do
  i <- lift $ unconditioned $ categorical [(0,0.25), (1,0.25), (2,0.5)]
  j <- lift $ unconditioned $ categorical [(i,0.25), (1,0.25), (2,0.5)]
  put (i + j)
  k <- get 
  return k

run_test1 = unsafePerformIO $ empiricalMeasure 10 (evalStateT test1 0) []

自定义MonadState实例

2 个答案: