用 Red 语言创建地图函数答案

【问题标题】：Creating map function in Red language用 Red 语言创建地图函数
【发布时间】：2017-10-15 08:17:41
【问题描述】：

如何用红色语言创建map，一个高阶函数。它应该将一个块和一个函数作为参数，并将发送的函数应用于块的每个成员。我尝试了以下代码：

Red []
mapfn: function[blk sfn][
    outblk: copy []
    foreach i blk[
        append outblk (sfn i) ]
    outblk ]

; to test: 
myblk: [" this " " is   " " a "  "    line " "for" "   testing " " only   "]
probe mapfn myblk 'reverse 
probe mapfn myblk 'trim

但它不起作用 - 它只是将原始块发回而不更改它或给出任何错误消息。如何纠正？

【问题讨论】：

标签： higher-order-functions rebol red

【解决方案1】：

在 Rebol 你有夹层函数 apply

>> help apply
USAGE:
    APPLY func block /only 

DESCRIPTION:
     Apply a function to a reduced block of arguments.
     APPLY is a function value.

ARGUMENTS:
     func -- Function value to apply (Type: any-function)
     block -- Block of args, reduced first (unless /only) (Type: block)

REFINEMENTS:
     /only -- Use arg values as-is, do not reduce the block

(SPECIAL ATTRIBUTES)
     throw

请参阅来源申请。

只要 Red 没有原生应用，你就可以编写自己的映射函数，例如

mapfn: function[blk sfn][
    outblk: copy []
    foreach i blk[
        append outblk sfn copy i 
    ]
    outblk 
]

使用 :functionname 获取函数

>> myblk: [" this " " is   " " a "  "    line " "for" "   testing " " only   "]
== [" this " " is   " " a " "    line " "for" "   testing " " only   "]
>> probe mapfn myblk :reverse 
[" siht " "   si " " a " " enil    " "rof" " gnitset   " "   ylno "]
== [" siht " "   si " " a " " enil    " "rof" " gnitset   " "   ylno "]
>> probe mapfn myblk :trim
["this" "is" "a" "line" "for" "testing" "only"]
== ["this" "is" "a" "line" "for" "testing" "only"]
>>

另一种更好的方法是因为您不能复制所有数据类型，例如

mapfn: function[blk sfn][
    collect [
        foreach i blk[
            keep sfn i 
        ]
    ]
]

如果不想修改原来的就这样调用函数

mapfn newblk: copy/deep myblk :reverse

【讨论】：

我改变了答案。 reverse 和 trim 在原始系列上工作并进行更改。所以你必须使用你工作的系列的副本。