A Common API to Modeling and Analysis Functions

A common interface is provided to allow users to specify a model without having to remember the different argument names across different functions or computational engines (e.g. 'R', 'Spark', 'Stan', etc).

One issue with different functions available in R that do the same thing is that they can have different interfaces and arguments. For example, to fit a random forest classification model, we might have:

rf_1 <- randomForest(x, y, mtry = 12, ntree = 2000, importance = TRUE)
 
# From ranger
rf_2 <- ranger(
  y ~ ., 
  data = dat, 
  mtry = 12, 
  num.trees = 2000, 
  importance = 'impurity'
)
 
# From sparklyr
rf_3 <- ml_random_forest(
  dat, 
  intercept = FALSE, 
  response = "y", 
  features = names(dat)[names(dat) != "y"], 
  col.sample.rate = 12,
  num.trees = 2000
)

Note that the model syntax is very different and that the argument names (and formats) are also different. This is a pain if you go between implementations.

In this example,

• the type of model is "random forest"
• the mode of the model is "classification" (as opposed to regression, etc).
• the computational engine is the name of the R package.

The idea of parsnip is to:

• Separate the definition of a model from its evaluation.
• Decouple the model specification from the implementation (whether the implementation is in R, spark, or something else). For example, the user would call rand_forest instead of ranger::ranger or other specific packages.
• Harmonize the argument names (e.g. n.trees, ntrees, trees) so that users can remember a single name. This will help across model types too so that trees will be the same argument across random forest as well as boosting or bagging.

Using the example above, the parsnip approach would be

rand_forest(mtry = 12, trees = 2000) %>%
  set_engine("ranger", importance = 'impurity') %>%
  fit(y ~ ., data = dat)

The engine can be easily changed and the mode can be determined when fit is called. To use Spark, the change is simple:

rand_forest(mtry = 12, trees = 2000) %>%
  set_engine("spark") %>%
  fit(y ~ ., data = dat)

To install it, use:

require(devtools)
install_github("tidymodels/parsnip")