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GHCi Prelude学习
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 11 :: 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 itPrelude> :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 -> aPrelude> :type namesnames :: [Char]Prelude> :type headhead :: [a] -> aPrelude> :type (head)(head) :: [a] -> a
再来看一个更加复杂的函数对象
Prelude> :type mapmap :: (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 modulesPrelude> :show contextPrelude> :show bindingsnames :: [Char] = "Daniel King"nums :: [Integer] = 1 : 2 : 3 : 4 : 5 : ....it :: [Integer] = 2 : 4 : 6 : 8 : 10 : ....Prelude> :show importsimport Prelude -- implicitPrelude> :show packagesactive package flags: nonePrelude> :show languagesbase language is: Haskell2010with the following modifiers: -XNoDatatypeContexts -XNondecreasingIndentation
Prelude> foldl ((+)) 0 [1..100]5050
Prelude> :type mapmap :: (a -> b) -> [a] -> [b]Prelude> :type filterfilter :: (a -> Bool) -> [a] -> [a]Prelude> :type foldrfoldr :: (a -> b -> b) -> b -> [a] -> bPrelude> :type foldlfoldl :: (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 foldrfoldr :: (a -> b -> b) -> b -> [a] -> bPrelude> foldr (:) "King" [‘D‘,‘a‘,‘n‘,‘i‘,‘e‘,‘l‘, ‘ ‘]"Daniel King"
但是用foldl就不行。
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