具有多态类型的STUArray

Question

我想使用STmonad和STUArrays 实现一个算法,我希望它能够同时使用Float和Double数据.

我将演示一个更简单的示例问题:计算memoized scanl (+) 0(我知道它可以解决STUArray,只是使用例子).

{-# LANGUAGE FlexibleContexts, ScopedTypeVariables #-}

import Control.Monad
import Control.Monad.ST
import Data.Array.Unboxed
import Data.Array.ST

accumST :: forall a. (IArray UArray a, Num a) => [a] -> Int -> a
accumST vals = (!) . runSTUArray $ do
  arr <- newArray (0, length vals) 0 :: ST s (STUArray s Int a)
  forM_ (zip vals [1 .. length vals]) $ \(val, i) ->
    readArray arr (i - 1)
    >>= writeArray arr i . (+ val)
  return arr

这失败了:

Could not deduce (MArray (STUArray s) a (ST s)) from the context ()
  arising from a use of 'newArray'
Possible fix:
  add (MArray (STUArray s) a (ST s)) to the context of
    an expression type signature
  or add an instance declaration for (MArray (STUArray s) a (ST s))

我不能应用建议的"可能的修复".因为我需要添加类似于(forall s. MArray (STUArray s) a (ST s))上下文的东西,但是afaik那是不可能的..

Answer 1

不幸的是，您当前无法创建要求未装箱数组可用于特定类型的上下文。不允许量化约束。但是，您仍然可以完成您想要做的事情（具有特定于类型的代码版本的额外优势。）对于较长的函数，您可以尝试拆分通用表达式，以便重复的代码尽可能小。

{-# LANGUAGE FlexibleContexts, ScopedTypeVariables #-}
module AccumST where 

import Control.Monad
import Control.Monad.ST
import Data.Array.Unboxed
import Data.Array.ST
import Data.Array.IArray

-- General one valid for all instances of Num.
-- accumST :: forall a. (IArray UArray a, Num a) => [a] -> Int -> a
accumST :: forall a. (IArray UArray a, Num a) => [a] -> Int -> a
accumST vals = (!) . runSTArray $ do
  arr <- newArray (0, length vals) 0 :: (Num a) => ST s (STArray s Int a)
  forM_ (zip vals [1 .. length vals]) $ \(val, i) ->
    readArray arr (i - 1)
    >>= writeArray arr i . (+ val)
  return arr

accumSTFloat vals = (!) . runSTUArray $ do
  arr <- newArray (0, length vals) 0 :: ST s (STUArray s Int Float)
  forM_ (zip vals [1 .. length vals]) $ \(val, i) ->
    readArray arr (i - 1)
    >>= writeArray arr i . (+ val)
  return arr

accumSTDouble vals = (!) . runSTUArray $ do
  arr <- newArray (0, length vals) 0 :: ST s (STUArray s Int Double)
  forM_ (zip vals [1 .. length vals]) $ \(val, i) ->
    readArray arr (i - 1)
    >>= writeArray arr i . (+ val)
  return arr

{-# RULES "accumST/Float" accumST = accumSTFloat #-}
{-# RULES "accumST/Double" accumST = accumSTDouble #-}

通用未装箱版本（不起作用）将具有如下类型约束：

accumSTU :: forall a. (IArray UArray a, Num a, 
    forall s. MArray (STUArray s) a (ST s)) => [a] -> Int -> a

您可以简化如下：

-- accumST :: forall a. (IArray UArray a, Num a) => [a] -> Int -> a
accumST :: forall a. (IArray UArray a, Num a) => [a] -> Int -> a
accumST vals = (!) . runSTArray $ do
  arr <- newArray (0, length vals) 0 :: (Num a) => ST s (STArray s Int a)
  accumST_inner vals arr

accumST_inner vals arr = do
  forM_ (zip vals [1 .. length vals]) $ \(val, i) ->
    readArray arr (i - 1)
    >>= writeArray arr i . (+ val)
  return arr

accumSTFloat vals = (!) . runSTUArray $ do
  arr <- newArray (0, length vals) 0 :: ST s (STUArray s Int Float)
  accumST_inner vals arr

accumSTDouble vals = (!) . runSTUArray $ do
  arr <- newArray (0, length vals) 0 :: ST s (STUArray s Int Double)
  accumST_inner vals arr

{-# RULES "accumST/Float" accumST = accumSTFloat #-}
{-# RULES "accumST/Double" accumST = accumSTDouble #-}