`subsetters` operators for `data.tables`

datatable_subsetters

`[[.data.table` <- function(data,subset){
  subset(data,eval(subset,data))
}

subsetter <- function(x){
  new_subsetter(substitute(x))
}

new_subsetter <- function(x){
    structure(x,class=c("subsetter","call"))
}

`%is%` <- function(lhs,rhs){
  lhs_nm <- deparse(substitute(lhs))
  rhs_2 <- substitute(rhs)[[2]]
  
  rhs_structured <- new_subsetter(rhs_2)
  
  assign(lhs_nm,
         rhs_structured,
         parent.frame())
}

`&.subsetter` <- function(a,b){
  new_subsetter(
    as.call(list(quote(`&`),a,b))
  )
}

`|.subsetter` <- function(a,b){
  new_subsetter(
    as.call(list(quote(`|`),a,b))
  )
}

`!.subsetter` <- function(x){
  new_subsetter(
    as.call(list(quote(`!`),x))
  )
}

print.subsetter <- function(x){
  cat("<unevaluated data.table subsetting operation>\n")
  cat(">> ",format(x))
}

flights14 <- fread("https://raw.githubusercontent.com/Rdatatable/data.table/master/vignettes/flights14.csv")

JFK %is% (origin == "JFK")
WITHIN_6_MONTHS %is% (month == 6L)
TO_LAX %is% (dest == "LAX")

jfk_flights_all <- flights14[[JFK]]

jfk_flights <- flights14[[JFK & WITHIN_6_MONTHS]]

jfk_not_lax <- flights14[[JFK & !TO_LAX]]