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。
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
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:cmd <expr> run the commands returned by <expr>::IO String
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(!: 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
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:!<command> run the shell command <command>
-- Commands for debugging:
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: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:
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: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。
原文：http://www.cnblogs.com/long123king/p/3838196.html