GHC 7.0.4 似乎忘记了如何应用函子

Question

在《Learn You A Haskell》的“函子、应用函子和幺半群”一章中，Miran 做了以下事情：

ghci> (pure 3) "blah"
3

然而我明白了：

ghci> (pure 3) "blah"
<interactive>:1:2:
    No instance for (Functor ((->) [Char]))
      arising from a use of `pure'
    Possible fix:
      add an instance declaration for (Functor ((->) [Char]))
    In the expression: (pure 3) "blah"
    In an equation for `it': it = (pure 3) "blah"

我不确定会发生什么。到目前为止，所有示例都运行正常。我肯定没有进口什么东西，但我不知道是什么。

这是我的版本信息：

$ ghci -v
GHCi, version 7.0.4: http://www.haskell.org/ghc/  :? for help
Glasgow Haskell Compiler, Version 7.0.4, for Haskell 98, stage 2 booted by GHC version 6.12.3

我的 ~/.ghc/ghci.conf 如下所示：

{-# LANGUAGE Arrows #-}

:set prompt "[32;1m%s[0m\n> "

import Control.Arrow
import Control.Monad (when, forever, forM, liftM)
import Control.Applicative
-- import Control.Applicative (ZipList (..), (<$>), (<*>), pure)
import Control.Exception (IOException (..), catch)
import qualified Data.ByteString as ByteString (pack, unpack)
import qualified Data.ByteString.Lazy as LazyByteString (cons, cons', empty, fromChunks, pack, readFile, unpack, writeFile)
import Data.Char (chr, ord, toUpper, toLower, isDigit)
import Data.Function (fix, on)
import Data.Functor
import Data.List (find, genericLength, intercalate, intersperse, nub, tails)
import Data.Map (Map (..))
import qualified Data.Map as Map (fromList, lookup)
import Data.Monoid (mempty, mappend, mconcat)
import Data.Tuple (fst, snd, curry, uncurry, swap)
import System.Console.ANSI (setCursorPosition, clearScreen)
import System.Directory (renameFile, doesFileExist)
import System.Environment (getArgs, getProgName)
import System.IO (IOMode (..), stdout, openFile, withFile, hGetContents, hClose, openTempFile, hPutStr, hFlush)
import System.IO.Error (isDoesNotExistError)
import System.Random (StdGen (..), RandomGen (..), Random (..), getStdGen, mkStdGen, newStdGen, random, randomR, randomRIO, randoms)
import Text.Printf (PrintfArg (..), printf)

Answer 1

您缺少 Control.Monad.Instances。不知道为什么它被定义在那里，但我以前也遇到过这个。

GHCi, version 7.0.4: http://www.haskell.org/ghc/  :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer-gmp ... linking ... done.
Loading package base ... linking ... done.

Prelude Control.Applicative> (pure 3) "blah"

<interactive>:1:2:
    No instance for (Functor ((->) [Char]))
      arising from a use of `pure'
    Possible fix:
      add an instance declaration for (Functor ((->) [Char]))
    In the expression: (pure 3) "blah"
    In an equation for `it': it = (pure 3) "blah"
Prelude Control.Applicative> import Control.Monad.Instances 
Prelude Control.Applicative Control.Monad.Instances> (pure 3) "blah"
3

另外，在查看 LYAH 中的章节后，作者确实定义了示例上方的实例，但不明显这已经在其他地方定义了。

更新

错误并不是因为 ghci 忘记了如何应用函子，而是函子实例 Functor ((->) [Char]) 尚未在您的环境中定义。