专业化使用GHC类型类

Question

我怎样才能让genOut /字符串火？

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：

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火？

Answer 1

它看起来像你想的重叠的情况下的，只要你想有一个instance Out String比instance Show a => Out a重叠它不同。

{-# 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

一般建议避免使用重叠实例和不可判定实例除非的真正必要的，因为改变类型检查他们做出不是便携式的，并可能导致其它问题。

---

修改

实施明智的，认为词典的一类的每个实例。 <<将给予字典，它有望作为隐藏参数使用Out的实例。由于out正在从那里抬头，就没有空间为您RULE火。如果在一个类型类out没有，而且正在从非多态函数调用，那么我预计RULE匹配，但是，它并不奇怪，这是行不通的。

RULE / SPECIALIZE意味着是唯一的优化，如果他们这样做或不会触发你的代码不应该改变自己的行为。

我花意识到你在做什么，一会儿......你知不知道Haskell是不是C ++，对不对？其中利用多态性的一种方法是完全不同的。