Explorations in functional programming

rentswatch.R

library(httr)
library(purrr)
library(dplyr)
library(ggplot2)
library(scales)
library(ggthemes)

with_msg <- function(msg, f) {
  
  function(...) {
    args <- list(...)
    message(msg, args[[1]])
    f(...)
  }
}

get_data <- function(city) {
  
  url <- paste0("http://api.rentswatch.com/api/cities/", city)
  response <- GET(url)
  stop_for_status(response)
  data <- content(response)
  
  data$months %>%
    map_df(as_data_frame) %>%
    mutate(month = as.Date(paste0(month, "-01")),
           city  = city) %>%
    rename("date" = month)
}

retrieve_data <- with_msg("Retrieving: ", safely(get_data))

cities <- c("Berlin", "Rome", "Madrid", "Milano", "Munich", "Brussels")

df <- cities %>% map(retrieve_data) %>% map_df("result")

# Retrieving: Berlin
# Retrieving: Rome
# Retrieving: Madrid
# Retrieving: Milano
# Retrieving: Munich
# Retrieving: Brussels

head(df)

# Source: local data frame [6 x 6]

#         date total avgPricePerSqm lastSnapshot     stdErr   city
#       (date) (int)          (dbl)        (int)      (dbl)  (chr)
# 1 2015-08-01  2037       12.20869   1463557537 0.08568070 Berlin
# 2 2015-09-01  1712       12.33864   1463557537 0.09864115 Berlin
# 3 2015-10-01  2320       11.77665   1463557537 0.08046912 Berlin
# 4 2015-11-01  3897       11.90312   1463557537 0.06291511 Berlin
# 5 2015-12-01  3424       12.16882   1463557537 0.07454032 Berlin
# 6 2016-01-01  2394       11.98097   1463557537 0.08729184 Berlin

ggplot(df, aes(x = date, y = avgPricePerSqm, group = city)) +
  geom_ribbon(aes(
    ymin = avgPricePerSqm - 2 * stdErr,
    ymax = avgPricePerSqm + 2 * stdErr,
    fill = city
  ),
  alpha = 0.2) +
  geom_line(aes(color = city)) +
  geom_text(
    aes(label = sprintf(" %s", city), color = city),
    data = filter(df, date == max(date)),
    size = 3,
    fontface = "bold",
    hjust = 0
  ) +
  scale_color_few() +
  scale_fill_few() +
  scale_y_continuous(breaks = pretty_breaks(10)) +
  scale_x_date(
    breaks = date_breaks("2 months"),
    labels = date_format("%b %Y", "CET"),
    expand = c(0, 0),
    limits = c(min(df$date), max(df$date) + 40)
  ) +
  theme_light() +
  theme(
    legend.position = "none",
    plot.caption = element_text(color = "grey50", size = rel(0.5)),
    plot.subtitle = element_text(color = "grey50"),
    plot.title = element_text(color = "grey30"),
    axis.text = element_text(size = rel(0.7))
  ) +
  labs(
    x = NULL,
    y = NULL,
    title = "Rents in major European citites",
    subtitle = "Average price per square meter (in Euros)",
    caption = "\nSource: Rentswatch.com"
  )