MODEL_FN() is a developer-interface constructor for user-facing model
functions like LINEAR_REG(). It returns a function that routes to the
correct implementation based on the variable mapper <var_id> and method variant.
Arguments
- cls
A string naming the model class, e.g.
"linear_reg".- defs
A list of
stat_define()objects.- .name
A string used as the model title in output.