Question

如何让genOut / String消防？

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：

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

如何解决这条规则？

Answer 1

您似乎想要overlapping instances，因为您希望instance Out String与instance Show a => Out a不同，而{-# 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与其重叠。

<<

一般建议是避免使用重叠实例和不可判断的实例，除非真正必要，因为他们所做的类型检查的更改不可移植并且可能导致其他问题。

修改

实现方面，考虑每个类实例的字典。 Out将为out的实例提供字典，预期它将用作隐藏参数。由于RULE正在从那里查找，因此out无法解雇。{1}}如果RULE不在类型类中，并且是从非多态函数调用的，那么我希望RULE匹配，但是它不会令人惊讶它不起作用

SPECIALIZE / {{1}}仅用于优化，如果代码执行或不执行，则代码不应更改行为。

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？利用多态的方式是完全不同的。

使用ghc在类型类中的特化

1 个答案:

修改