Possible cdo wrapper in R.

cdor.R

# This is an experiment to wrap CDO in R in a intuitive and pipe friendly way. 
# Usage:
#  cdo_* will create a string with a commmand. If any of the "input_file" arguments
#  is a cdo command, it will append "-" to it so that it can be chained. 
#  This is good for efficiency as it will allow you to apply multiple 
#  operations without having to write to disk the intermediate files. 


## "Internal" functions in which the magic happens
.enchain <- function(input) {
  if (inherits(input, "cdo_output")) {
    stop("This operation has output and thus cannot be chained:\n", input)
  }
  
  if (inherits(input, "cdo_operator")) {
    input <- paste0("-", input)
  }
  
  return(input)
}

.build_command <- function(operator, inputs, outputs = NULL) {
  if (!is.null(outputs)) {
    command <- paste0("cdo ", operator, " ",
                      paste(inputs, collapse = " "), " ",
                      paste(outputs, collapse = " "))
    class(command) <- c("cdo_output", class(command)) 
    return(command)
  } else {
    command <- paste0(operator, " ",
                      paste(inputs, collapse = " "))
    class(command) <- c("cdo_operator", class(command))
    return(command)
    
  }
}


# Define a couple of operators.
# This has to be done for the ~700 operators. Ideally, automatically.
cdo_sub <- function(input_file_1, input_file_2, output_file = NULL) {
  input_file_1 <- .enchain(input_file_1)
  input_file_2 <- .enchain(input_file_2)
  
  .build_command("sub", c(input_file_1, input_file_2), output_file)
}



cdo_dayavg <- function(input_file, output_file = NULL) {
  input_file <- .enchain(input_file)
  
  .build_command("dayavg", input_file, output_file)
}

cdo_timavg <- function(input_file, output_file = NULL) {
  input_file <- .enchain(input_file)
  
  .build_command("timavg", input_file, output_file)
}


cdo_abs <- function(input_file, output_file = NULL) {
  input_file <- .enchain(input_file)
  
  .build_command("timabs", input_file, output_file)
}


cdo_output <- function(command, output_file) {
  paste0("cdo ", command, " ", output_file)
}


cdo_run <- function(command) {
  system(command)
}


## Mock examples (should end with `cdo_run()` to be performed)
library(magrittr)

cdo_sub(input_file_1 = cdo_dayavg("file1"), 
        input_file_2 = cdo_timavg("file2"), 
        output_file = "file3")


"file" %>% 
  cdo_dayavg() %>% 
  cdo_abs() %>% 
  cdo_output("file2")
  

# Error
cdo_dayavg("file", "outfile") %>% 
  cdo_abs()