1. A system, comprising: one or more computing devices configured to: receive, at a network-accessible machine learning service of a provider network, a text representation of a recipe comprising one or more of: (a) a group definitions section indicating one or more groups of variables, wherein individual ones of the one or more groups comprise a plurality of variables on which at least one common transformation operation is to be applied, (b) an assignment section defining one or more intermediate variables, (c) a dependency section indicating respective references to one or more machine learning artifacts stored in a repository, or (d) an output section indicating one or more transformation operations to be applied to at least one entity indicated in the group definitions section, the assignment section, or the dependency section; validate, in accordance with (a) a set of syntax rules defined by the machine learning service and (b) a set of library function definitions for transformation operation types supported by the machine learning service, the text representation of the recipe; generate an executable representation of the recipe; store the executable representation in the repository; determine that the recipe is to be applied to a particular data set; verify that the particular data set meets a run-time acceptance criterion of the recipe; and apply, using one or more selected provider network resources, a particular transformation operation of the one or more transformation operations to the particular data set.