strategy-pattern.R

#!/usr/bin/env Rscript

cat("==========================================================================\n")
cat("Example of strategy patern in R: Multiple ways to show routes. \n")

# Problem:
# You build a navigation app for casual travelers and help users quickly orient themselves in any city.
# 
# - The first only build the routes over roads.
# - Next update, you added an option to build walking routes. 
# - Right after that, you added another option to let people use public transport in their routes.
# - Later you planned to add route building for cyclists. 
# 
# Each time you added a new routing algorithm, the navigator function increased the size.
# Any change to one of the algorithms, affected the whole function, increasing the chance of creating an error in already-working code.
# Implementing a new feature requires you to change the same huge function, conflicting with the code produced by other people.
# 
# Solution in R:
# Take a function with a lot of conditionals or branchs and move them into separate implementations.
# We can use generic functions to write strategy implementations in each concrete method of each strategy.
# 
# The main function would be isn’t responsible for selecting an appropriate algorithm for the job.
# In fact, the main function doesn’t know much about strategies.
# Is the client who passes the object as argument with the desired strategy for the generic function.
# 
# In our navigation app each routing algorithm can be extracted to its buildRoute implementation.
# Even though given the same arguments, each routing class might build a different route, the main navigator function.
# doesn’t really care which algorithm is selected since its primary job is to get a set of checkpoints on the map.
# The class has a property for switching the active routing strategy, so its clients in the
# user interface, can replace the currently selected routing behavior with another one.

select_route_strategy <- \() {
    strategy_class <- ""
    while(strategy_class == "") {
        cat("\nWhat option do you want?\nHere are the options: 1) roads 2) walk.\n");
        strategy_selection <- readLines("stdin", n = 1);
        strategy_class <- dplyr::case_match(
            strategy_selection,
            "1" ~ "routes_roads",
            "2" ~ "routes_walking",
            .default = ""
        )
    }
    return(structure(list(), class = strategy_class))
}
new_context <- \(data, routes_strategy = NULL) {
    structure(list(data = data, routes_strategy = routes_strategy), class = "context")
}
buildRoute <-  \(x, context) {
    UseMethod("buildRoute")
}
buildRoute.default <- \(x, context) {
    stop("You must select a valid route_strategy")
}
buildRoute.routes_roads <- \(x, context) {
    subset(context$data,  y == 1) 
}
buildRoute.routes_walking <- \(x, context) {
    subset(context$data,  y == 2) 
}
navigator <- \(context){
    buildRoute(context$routes_strategy, context)
}

cat("Give me 4 numbers:\n");
x <- readLines("stdin", n = 4);
df <- data.frame(x = x, y = c(1, 1, 2, 2))

repeat {
    route_strategy <- select_route_strategy()
    context <- new_context(
        data = df,
        routes_strategy = route_strategy
    )
    print(navigator(context))
}