参考：http://www.cse.unsw.edu.au/~en1000/haskell/inbuilt.html

http://www.cse.unsw.edu.au/~en1000/haskell/hof.html

在GHCi中，可以使用:type来查看对象的类型，与http://www.cnblogs.com/long123king/p/3837686.html中说到的一样，

Haskell中，函数也是一种特殊的对象，对象就有类型，函数作为一种对象，可以作为参数传递，也可以赋值，创建和销毁。

Prelude> :type (+)
(+) :: Num a => a -> a -> a

这个类似要怎么解释呢，"::"的前面是函数的名称，后面是函数对象的类型，或者说原型。

"=>"前面的Num a是表明参数的类型，

Prelude> :type 1
1 :: Num a => a

后面是函数的输入与输出类型声明。

之所以会有多个->，那是因为(+)函数对象中其实包含一个更加简单的函数，比如(+) 2，这个函数的意思是“在使用(+)函数对象时，将第一个参数固定为2，这与boost中的bind类似”，而这个函数对象的类型是Num a => a->a，

再把另外一个参数传递给这个简单的函数，得到的结果也是a类型，因此(+)是一个复合函数。

凡是需要多个参数的函数对象，都可以分解成一步一步的简单函数组成的复合函数。

Prelude> (+) 2

<interactive>:35:1:
    No instance for (Num a0) arising from a use of `+‘
    The type variable `a0‘ is ambiguous
    Possible fix: add a type signature that fixes these type variable(s)
    Note: there are several potential instances:
      instance Num Double -- Defined in `GHC.Float‘
      instance Num Float -- Defined in `GHC.Float‘
      instance Integral a => Num (GHC.Real.Ratio a)
        -- Defined in `GHC.Real‘
      ...plus three others
    In the expression: (+) 2
    In an equation for `it‘: it = (+) 2

<interactive>:35:1:
    No instance for (Show (a0 -> a0)) arising from a use of `print‘
    Possible fix: add an instance declaration for (Show (a0 -> a0))
    In a stmt of an interactive GHCi command: print it
Prelude> :type (+) 2
(+) 2 :: Num a => a -> a

特殊符号需要显式地用括号来表明这是个函数对象，

对于普通的函数对象，也可以使用括号来表明其函数对象的身份。

Prelude> :type +

<interactive>:1:1: parse error on input `+‘
Prelude> :type (+)
(+) :: Num a => a -> a -> a
Prelude> :type names
names :: [Char]
Prelude> :type head
head :: [a] -> a
Prelude> :type (head)
(head) :: [a] -> a

再来看一个更加复杂的函数对象

Prelude> :type map
map :: (a -> b) -> [a] -> [b]

这个函数对象包含了两个简单函数对象， (a -> b)是一个函数，可以将类型a的对象转换成类型b的对象；

(a -> b) -> [a]，是另外一个函数，它的意思是“在执行整体函数对象时，将第一个参数固定为[a]”。

这种将复合函数对象（包含了多个参数的函数对象）分解成几个简单函数对象的思想，是为了支持“函数作为一种对象”的这种设计理念，

这样就可以将上面分解出的简单的函数作为参数，传递给复合函数对象。

比如

Prelude> let nums = [1..100]
Prelude> map ((*) 2) nums
[2,4,6,8,10,12,14,16,18,20,22,24,26,28,30,32,34,36,38,40,42,44,46,48,50,52,54,56,58,60,62,64,66,68,70,72,74,76,78,80,82,84,86,88,90,92,94,96,98,100,102,104,106,108,110,112,114,116,118,120,122,124,126,128,130,132,134,136,138,140,142,144,146,148,150,152,154,156,158,160,162,164,166,168,170,172,174,176,178,180,182,184,186,188,190,192,194,196,198,200]

虽然，像(+) 2这样的函数对象在多数时候，看起来并不是一个常见的用法。

怎样在Prelude中得到当前运行环境的信息

Prelude> :help
 Commands available from the prompt:

   <statement>                 evaluate/run <statement>
   :                           repeat last command
   :{\n ..lines.. \n:}\n       multiline command
   :add [*]<module> ...        add module(s) to the current target set
   :browse[!] [[*]<mod>]       display the names defined by module <mod>
                               (!: more details; *: all top-level names)
   :cd <dir>                   change directory to <dir>
   :cmd <expr>                 run the commands returned by <expr>::IO String
   :ctags[!] [<file>]          create tags file for Vi (default: "tags")
                               (!: use regex instead of line number)
   :def <cmd> <expr>           define command :<cmd> (later defined command has
                               precedence, ::<cmd> is always a builtin command)
   :edit <file>                edit file
   :edit                       edit last module
   :etags [<file>]             create tags file for Emacs (default: "TAGS")
   :help, :?                   display this list of commands
   :info [<name> ...]          display information about the given names
   :issafe [<mod>]             display safe haskell information of module <mod>
   :kind <type>                show the kind of <type>
   :load [*]<module> ...       load module(s) and their dependents
   :main [<arguments> ...]     run the main function with the given arguments
   :module [+/-] [*]<mod> ...  set the context for expression evaluation
   :quit                       exit GHCi
   :reload                     reload the current module set
   :run function [<arguments> ...] run the function with the given arguments
   :script <filename>          run the script <filename>
   :type <expr>                show the type of <expr>
   :undef <cmd>                undefine user-defined command :<cmd>
   :!<command>                 run the shell command <command>

 -- Commands for debugging:

   :abandon                    at a breakpoint, abandon current computation
   :back                       go back in the history (after :trace)
   :break [<mod>] <l> [<col>]  set a breakpoint at the specified location
   :break <name>               set a breakpoint on the specified function
   :continue                   resume after a breakpoint
   :delete <number>            delete the specified breakpoint
   :delete *                   delete all breakpoints
   :force <expr>               print <expr>, forcing unevaluated parts
   :forward                    go forward in the history (after :back)
   :history [<n>]              after :trace, show the execution history
   :list                       show the source code around current breakpoint
   :list identifier            show the source code for <identifier>
   :list [<module>] <line>     show the source code around line number <line>
   :print [<name> ...]         prints a value without forcing its computation
   :sprint [<name> ...]        simplifed version of :print
   :step                       single-step after stopping at a breakpoint
   :step <expr>                single-step into <expr>
   :steplocal                  single-step within the current top-level binding
   :stepmodule                 single-step restricted to the current module
   :trace                      trace after stopping at a breakpoint
   :trace <expr>               evaluate <expr> with tracing on (see :history)

 -- Commands for changing settings:

   :set <option> ...           set options
   :seti <option> ...          set options for interactive evaluation only
   :set args <arg> ...         set the arguments returned by System.getArgs
   :set prog <progname>        set the value returned by System.getProgName
   :set prompt <prompt>        set the prompt used in GHCi
   :set editor <cmd>           set the command used for :edit
   :set stop [<n>] <cmd>       set the command to run when a breakpoint is hit
   :unset <option> ...         unset options

  Options for ‘:set‘ and ‘:unset‘:

    +m            allow multiline commands
    +r            revert top-level expressions after each evaluation
    +s            print timing/memory stats after each evaluation
    +t            print type after evaluation
    -<flags>      most GHC command line flags can also be set here
                         (eg. -v2, -fglasgow-exts, etc.)
                    for GHCi-specific flags, see User‘s Guide,
                    Flag reference, Interactive-mode options

 -- Commands for displaying information:

   :show bindings              show the current bindings made at the prompt
   :show breaks                show the active breakpoints
   :show context               show the breakpoint context
   :show imports               show the current imports
   :show modules               show the currently loaded modules
   :show packages              show the currently active package flags
   :show language              show the currently active language flags
   :show <setting>             show value of <setting>, which is one of
                                  [args, prog, prompt, editor, stop]
   :showi language             show language flags for interactive evaluation

Prelude> :show modules
Prelude> :show context

Prelude> :show bindings
names :: [Char] = "Daniel King"
nums :: [Integer] = 1 : 2 : 3 : 4 : 5 : ....
it :: [Integer] = 2 : 4 : 6 : 8 : 10 : ....
Prelude> :show imports
import Prelude -- implicit
Prelude> :show packages
active package flags: none
Prelude> :show languages
base language is: Haskell2010
with the following modifiers:
  -XNoDatatypeContexts
  -XNondecreasingIndentation

Prelude> foldl ((+)) 0 [1..100]
5050

Prelude> :type map
map :: (a -> b) -> [a] -> [b]
Prelude> :type filter
filter :: (a -> Bool) -> [a] -> [a]
Prelude> :type foldr
foldr :: (a -> b -> b) -> b -> [a] -> b
Prelude> :type foldl
foldl :: (a -> b -> a) -> a -> [b] -> a

Prelude> filter ((>) 50) nums
[1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49]

Prelude> :type foldr
foldr :: (a -> b -> b) -> b -> [a] -> b
Prelude> foldr (:) "King" [‘D‘,‘a‘,‘n‘,‘i‘,‘e‘,‘l‘, ‘ ‘]
"Daniel King"

但是用foldl就不行。

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