问题
How can I create map, a higher order function, in Red language. It should take a block and a function as arguments and apply the sent function to each member of block. I tried following code:
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
But it is not working - it simply sends back the original block without changing it or giving any error message. How can this be corrected?
回答1:
In Rebol you have the mezzanine function 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
See source apply.
As long as Red has no native apply you can write your own mapping function e.g
mapfn: function[blk sfn][
outblk: copy []
foreach i blk[
append outblk sfn copy i
]
outblk
]
Get the function with :functionname
>> 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"]
>>
An alternative and better way as you can not copy all datatypes is e.g.
mapfn: function[blk sfn][
collect [
foreach i blk[
keep sfn i
]
]
]
and call the function this way if no do not want to modify the original
mapfn newblk: copy/deep myblk :reverse
来源：https://stackoverflow.com/questions/46753092/creating-map-function-in-red-language