Implement more generic functions for Pipe object

[renkun-ken]

For example, head(), tail(), etc. should directly work on $value rather than Pipe environment.

[renkun-ken]

head.Pipe and tail.Pipe does not pass R CMD check.

[renkun-ken]

[ ], [[ ]], $<- are defined for Pipe object so that the pipe continues with subsetting and extracting, as well as element setting. For example,

> Pipe(list(a=1))["a"]
$value : list 
------
$a
[1] 1
> Pipe(list(a=1,b=2))[c("a","b")]
$value : list 
------
$a
[1] 1

$b
[1] 2
> Pipe(list(a=1,b=2))[["a"]]
$value : numeric 
------
[1] 1
> Pipe(mtcars)["mpg"]$head()
$value : data.frame 
------
                   mpg
Mazda RX4         21.0
Mazda RX4 Wag     21.0
Datsun 710        22.8
Hornet 4 Drive    21.4
Hornet Sportabout 18.7
Valiant           18.1
> Pipe(mtcars)$as.data.table()[,mpg]
$value : numeric 
------
 [1] 21.0 21.0 22.8 21.4 18.7 18.1 14.3 24.4 22.8 19.2 17.8 16.4 17.3 15.2 10.4
[16] 10.4 14.7 32.4 30.4 33.9 21.5 15.5 15.2 13.3 19.2 27.3 26.0 30.4 15.8 19.7
[31] 15.0 21.4

It works with data.table too.

> z <- data.table(x=1:3,y=rnorm(3),key = "x")
> p <- Pipe(z)
> p[J(1)]
$value : data.table data.frame 
------
   x         y
1: 1 0.1901827
```rconsole
> p[J(1), y:=0.5]
$value : data.table data.frame 
------
   x          y
1: 1  0.5000000
2: 2  1.0253236
3: 3 -0.6331669

Element setting also works.

> p$y <- 1:3
> p
$value : data.table data.frame 
------
   x y
1: 1 1
2: 2 2
3: 3 3

[renkun-ken]

This allows direct call of subsetting and extracting, for example,

Pipe(mtcars)$
  as.data.table()[mpg <= mean(mpg)]$
  .(lm(mpg ~ wt + cyl, data = .))$
  summary()$
  .(coefficients)

$value : matrix 
------
             Estimate Std. Error   t value     Pr(>|t|)
(Intercept) 31.928632  4.0100333  7.962186 9.127888e-07
wt          -2.183321  0.6536109 -3.340398 4.472574e-03
cyl         -1.012133  0.5699081 -1.775959 9.602413e-02