2

我怎样才能让 genOut/String 着火？

module IOStream where

import System.IO
import System.IO.Unsafe

class Out a where
  out :: a → String

instance Show a ⇒ Out a where
  out = show

outString :: String → String
outString = id
{-# RULES "genOut/String" out = outString #-}

infixl 9 <<, ≪
(≪), (<<) :: Out a ⇒ IO Handle → a → IO Handle
(<<)= (≪)
h ≪ a = do
  s ← h
  hPutStr s $ out a
  return s

cout, cin, cerr :: IO Handle
cout = return stdout
cin  = return stdin
cerr = return stderr

endl :: String
endl = "\n"

--infixr 9 ∘ °
(∘) = (.)
(°) = flip (∘)

module Main where

import System.IO
import IOStream

foreign import ccall "pi.h f_" f_ :: IO Double

main :: IO Int
main = do
  --putStrLn . show =<< f_

--  ((≪ endl) . (cout ≪)) =<< f_ 
  (cout ≪) ° (≪ endl) =<< f_
  return 0

编译和链接：

cetin@unique:~/lab/c/linking/demo$ ghc --version
The Glorious Glasgow Haskell Compilation System, version 6.10.2
cetin@unique:~/lab/c/linking/demo$ ghc -fglasgow-exts -O2 -ddump-simpl-stats -XUndecidableInstances -O2 iostream.hs main.hs pi_v2.o -o hsex2

==================== FloatOut stats: ====================
0 Lets floated to top level; 1 Lets floated elsewhere; from 3 Lambda groups



==================== FloatOut stats: ====================
0 Lets floated to top level; 0 Lets floated elsewhere; from 5 Lambda groups



==================== Grand total simplifier statistics ====================
Total ticks:     184

40 PreInlineUnconditionally
45 PostInlineUnconditionally
24 UnfoldingDone
8 LetFloatFromLet
4 EtaReduction
57 BetaReduction
6 KnownBranch
11 SimplifierDone



==================== FloatOut stats: ====================
0 Lets floated to top level; 0 Lets floated elsewhere; from 1 Lambda groups



==================== FloatOut stats: ====================
3 Lets floated to top level; 0 Lets floated elsewhere; from 1 Lambda groups



==================== Grand total simplifier statistics ====================
Total ticks:     218

42 PreInlineUnconditionally
57 PostInlineUnconditionally
33 UnfoldingDone
9 LetFloatFromLet
1 EtaReduction
66 BetaReduction
10 KnownBranch
12 SimplifierDone

结果：

cetin@unique:~/lab/c/linking/demo$ ./hsex2
3.141592653589793"\n"cetin@unique:~/lab/c/linking/demo$

预期的：

cetin@unique:~/lab/c/linking/demo$ ./hsex2
3.141592653589793
cetin@unique:~/lab/c/linking/demo$

我怎样才能让这条规则着火？

4

1 回答 1

4

看起来你想要重叠的实例，因为你想要一个与它重叠instance Out String的不同。instance Show a => Out a

{-# LANGUAGE FlexibleInstances, TypeSynonymInstances #-}
{-# LANGUAGE OverlappingInstances, UndecidableInstances #-}
class Out a where out :: a -> String
instance Out String where out = id
instance (Show a) => Out a where out = show

一般建议是避免使用重叠实例和不可判定实例，除非确实有必要，因为它们对类型检查所做的更改是不可移植的，并且可能导致其他问题。

编辑

在实现方面，请考虑类的每个实例的字典。 <<将给出字典作为Out它预期用作隐藏参数的实例。既然out是从那里向上看，你RULE就没有开火的余地了。如果out不在类型类中，并且从非多态函数中调用，那么我希望RULE匹配，但事实上，它不起作用也就不足为奇了。

RULE/SPECIALIZE仅用于优化，如果它们触发或不触发，您的代码不应改变行为。

我花了一段时间才意识到你在做什么……你确实意识到 Haskell 不是 C++，对吧？利用多态性的方式完全不同。

于 2009-06-05T14:00:29.867 回答